Trace-flag display reads anchor on the collector's newest successful run (#3999)

[erikdarlingdata]

Closes #3999.

Why

get_trace_flags (MCP, DarlingCurrentConfigReader.TraceFlagsSql), the web viewer's Trace Flags grid
(ViewerDataService.Config.TraceFlagsSql), and Lite's equivalent (LocalDataService.Config.cs
GetLatestTraceFlagsAsync) all anchored on capture_time = (SELECT MAX(capture_time) FROM v_trace_flags WHERE server_id = $1). This is correct for the ordinary case (one flag turns off while others stay on:
the off flag's stale row has an OLDER capture_time than the new MAX, so it is excluded correctly). It
fails on the edge case #3929's background also named for the analysis fact: a capture that finds EVERY
flag off writes ZERO rows (DBCC TRACESTATUS(-1) only ever lists flags that are ON), so MAX(capture_time)
cannot see that capture at all and silently falls back to an older capture that still had a flag on,
reporting a flag as currently enabled days or weeks after it (and every other flag) was turned off.

Erik's ruling (posted on the issue): anchor the display reads on the trace_flags collector's newest
SUCCESSFUL run in collection_log, not on the newest row. If that run is newer than the newest
trace_flags row, no flag is on, making the all-off case exact.

What changes

Each of the three reads gained a second AND: the newest trace_flags row's own timestamp must be
>= the newest SUCCESS this collector logged in v_collection_log. When a later successful run
found nothing on, that comparison is false, the whole WHERE matches no row, and the read correctly
reports zero flags instead of falling back to a stale one.

AND   (SELECT MAX(capture_time) FROM v_trace_flags WHERE server_id = $1) >= COALESCE(
          (SELECT MAX(collection_time) FROM v_collection_log
           WHERE server_id = $1 AND collector_name = trace_flags AND status = SUCCESS),
          TIMESTAMP 1900-01-01)

(quoted string literals in the actual code; shown unquoted above only to keep this body's markdown
simple to render)

The COALESCE floor matters only when collection_log's retention has aged past this collector's oldest
trace_flags row, or nothing has run yet: with no successful run to corroborate against, it falls back
to the pre-fix reading rather than wrongly suppressing a row it cannot compare.

• Darling/PerformanceMonitor.Darling.Service/Mcp/DarlingCurrentConfigReader.cs - TraceFlagsSql
  (get_trace_flags).
• Darling/PerformanceMonitor.Darling.Viewer/ViewerDataService.Config.cs - TraceFlagsSql (the web
  viewer's Trace Flags grid).
• Lite/Services/LocalDataService.Config.cs - GetLatestTraceFlagsAsync (Lite parity, see below).
• The analysis fact (PgFactCollector.Config.cs's TraceFlagsSql, and Lite's
  DuckDbFactCollector.Config.cs twin) is not touched. It already uses a different, independently
  correct technique from On-load config collectors also recapture daily, not just on connect (#3929) (#3930) #4001 (an on-load-aware lookback WINDOW rather than an exact match), which the
  issue's own ruling only asked to revisit "if it lets the window fallback go", a conditional
  simplification of already-correct code, not a defect, and out of scope for a hard-deadline lane.
  Flagging as a possible small follow-up rather than doing it speculatively under time pressure.

Lite parity

Checked Lite/Services/LocalDataService.Config.cs GetLatestTraceFlagsAsync against both Darling
reads: byte-identical defect (DuckDB SQL, same capture_time = MAX(capture_time) shape, same
v_trace_flags / v_collection_log view names). Fixed with the identical additional AND clause, DuckDB
syntax being PostgreSQL-compatible here (COALESCE, scalar subquery, TIMESTAMP literal all work
unchanged).

Test plan

• Darling.Tests builds with 0 Warning(s) (includes the new live test).
• Lite.Tests not rebuilt as part of this PR's local run, see below. The Lite change is a
  3-line SQL addition mirroring the already-verified Darling SQL byte-for-byte in shape, so I judged
  the risk low against the remaining time budget. Coordinator/CI should confirm Lite.Tests builds
  clean; flagging rather than asserting it silently.
• New live regression test, Darling.Tests/ViewerConfigurationTests.cs:
  TraceFlags_AllOffSinceNewerSuccessfulRun_ReadsNoFlags_AgainstDevPostgres. Seeds one trace flag
  ON two days ago, then a collection_log SUCCESS for trace_flags one day after that (simulating the
  daily recapture that found nothing on and wrote no row), and asserts GetLatestTraceFlagsAsync
  returns EMPTY. This is exactly the bug shape: before the fix this would have returned the
  two-day-stale ON flag.
• Existing live test TraceFlags_LatestCaptureWins_OrderedByFlag_AgainstDevPostgres re-read: it
  plants no collection_log row at all, so the COALESCE floor keeps it passing unchanged; the
  ordinary multi-flag case is untouched.
• Existing SQL-substring pins (ViewerConfigurationTests.TraceFlagsSql_SelectsSnapshotColumns...,
  DarlingMcpConfigToolsTests.TraceFlagsSql_LatestSnapshot) use Assert.Contains, not an exact
  match, against the original capture_time = (SELECT MAX...) clause I preserved verbatim ahead of
  the new AND; re-read to confirm they still hold, not just assumed.
• Re-read the other two live fixtures that plant trace_flags rows with no matching collection_log
  row (DarlingMcpConfigToolsTests.ConfigTools_ReadPlantedSnapshots_AgainstDevPostgres,
  McpLatestSnapshotStampTests config section); both stay correct under the COALESCE floor for the
  same reason.
• Targeted class run (build machine has no live Postgres, so live facts self-skip via
  Assert.SkipWhen inside the run rather than being excluded by filter): Darling.Tests filtered to
  ViewerConfigurationTests, DarlingMcpConfigToolsTests, McpLatestSnapshotStampTests: 35 total, 0
  failed.
• The new live test has NOT been executed against a real Postgres. This lane had no time left in
  its 13:10Z window to stand up the PG rig after On-load collectors are now staleness-exempt forever in CollectorHealthClassifier, even though they recapture daily #4000. Written and reasoned through carefully (see
  "Why" above) but UNVERIFIED live. This is the single most important thing for the coordinator to
  check before treating this PR as done; please run it against the CI darling-pg job or the local
  rig before arming auto-merge.
• No dedicated Lite/DuckDB live regression test added for the all-off case, for the same reason.
  The source fix mirrors Darling's exactly; a DuckDB equivalent of the new live test (using
  SharedDuckDbFixture, following McpLatestSnapshotStampTests.cs's pattern) is the natural follow-up
  if the coordinator wants matching live coverage on both SKUs.

What the coordinator should double-check

• Run the new live test for real (TraceFlags_AllOffSinceNewerSuccessfulRun_ReadsNoFlags_AgainstDevPostgres);
  it is reasoned through but not executed, per the Test plan above.
• Lite.Tests build was not re-run locally after the LocalDataService.Config.cs edit; low risk (an
  additive SQL clause plus one doc comment, same shape as the verified Darling changes) but not proven.
• Whether the analysis fact's window-based TRACE_FLAGS read (On-load config collectors also recapture daily, not just on connect (#3929) (#3930) #4001) is worth simplifying onto this same
  collection_log anchor, per the issue's own "if it lets the window fallback go"; left untouched here.
• No store migration; this is a SQL-clause change against existing tables/views, no schema change.

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