fix(#465): validate a dashboard-variable tab's Run as a variable option query

[BorisTyshkevich]

What & why

Closes #465.

#457 moved a Dashboard variable's option SQL into a dashboard-variable
main-editor tab, but deleted the drawer's Test action along with it — Run
executed the SQL raw and reported a plain success even when it returned the
wrong number or type of columns. The break surfaced later and further away:
only when the Dashboard-wide option batch failed, as one merged UNION ALL
column list that can't say which variable's branch was at fault.

Run on a dashboard-variable tab is now validated as a variable option query
end to end:

• Every locally-detectable problem (blank/comment-only SQL, non-SELECT,
  more than one statement, FORMAT/INTO OUTFILE, a {name:Type} parameter,
  an optional /*[ … ]*/ block, an unterminated string/comment, the reserved
  batch-tag column) is reported with no request sent, before authentication —
  and a multi-statement variable query is rejected by name rather than
  silently routed to the ordinary script runner.
• A query that passes runs through the same bounded single-branch probe
  (compileOptionProbe) the option batch's own branches use, so Run can never
  accept SQL the batch would reject.
• The response is then checked against the same shape the batch requires —
  exactly two non-nullable String-compatible columns (String,
  LowCardinality(String), FixedString(N)) — and a shape failure is never
  recorded as a successful run (no history entry, no bound-parameter
  recording, no detached-result source, no lastSuccessfulResultColumns
  update).
• Ordinary query-tab Run behaviour is unchanged.

A review pass on this branch caught a second issue before it shipped: the
Explain button/view-switch would have silently ignored Explain on a variable
tab (dispatching straight into the new validated Run path with no indication
anything was ignored) instead of running it. explainQuery/setExplainView
now refuse with a toast, mirroring the existing multi-statement-blocked
pattern.

See docs/ADR-0003-dashboard-viewing.md's #465 addendum for the full design
rationale (why the preflight lives ahead of the run()/runScript() split,
why a variable document still isn't a query, etc.).

Test plan

• npm test (100/100/100/100 gate + tsc --noEmit) — green
• npm run build — green
• npm run test:e2e (chromium + webkit) — green, including two new Run
  cases (valid query displays rows; shape-invalid query reports the
  problem, never a successful result) added to the existing
  variable-tab fixture
• Unit tests: restored compileOptionProbe/isOptionColumnType/
  validateOptionColumns in core/variable-options.test.ts;
  WorkbenchSession's new variable Run path (every acceptance-criteria
  bullet) in workbench-session.test.ts; the Explain-guard fix in
  app.test.ts

Checklist

• npm test passes (the per-file coverage gate is non-negotiable)
• Tests added/updated in the same change as the code
• npm run build succeeds (single-file dist/sql.html)
• Layers kept honest: pure logic in src/core/, network in src/net/ (injected fetch), DOM in src/ui/
• No new runtime dependency
• CHANGELOG.md ([Unreleased]) updated
• Reconciled affected tracked work: roadmap Roadmap to 1.0.0 #68 (#457's follow-up line),
  docs/ADR-0003-dashboard-viewing.md (new #465 addendum)

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