Say why CommitDelayedDiagnostics copies the collection

[xperiandri]

Description

Comment and local-name only change in CapturingDiagnosticsLogger.CommitDelayedDiagnostics. No behaviour change.

member _.CommitDelayedDiagnostics(diagnosticsLogger: DiagnosticsLogger) =
    // A sink can report back into this logger while replaying, so iterate a snapshot.
    let snapshot = diagnostics.ToArray()
    snapshot |> Array.iter diagnosticsLogger.DiagnosticSink

The old comment ("Eagerly grab all the errors and warnings from the mutable collection") said that the copy is eager but not what it protects against, and errors named a collection that also holds warnings. Together they read like a gratuitous copy of a ResizeArray that is consumed in full, immediately, by the very next line — an inviting target for "just iterate it lazily".

It is not gratuitous. A sink can report a diagnostic back into the logger being replayed, and a lazy traversal would then die with Collection was modified; enumeration operation may not execute:

• fsc.fs installs delayForFlagsLogger as the thread's logger (fsc.fs#L498) and never unwinds it across the commits that follow. Each of those builds a console logger whose DiagnosticSink calls TcConfig.Create(tcConfigB, validate = false) (fsc.fs#L78), which reports through the ambient logger — still the CapturingDiagnosticsLogger mid-replay.
• RunWithBufferedReporting commits from its exception path while use _restoreLogger = UseDiagnosticsLogger capturingLogger is still in scope (NameResolution.fs#L2594).

With the snapshot, a diagnostic reported during a replay simply stays in the collection for the next commit. Without it, the failure is an InvalidOperationException far from its cause and only on the re-entrant paths.

Fixes # (issue, if applicable) — none.

Checklist

• Test cases added — not applicable; nothing observable changed. FSharp.Compiler.Service.Tests.BuildGraphTests (15 tests, the existing coverage for CommitDelayedDiagnostics via MultipleDiagnosticsLoggers.Parallel/Sequential, including diagnostic ordering) passes.
• Performance benchmarks added in case of performance changes — not applicable.
• Release notes entry updated — not applicable, no user-visible change. Please apply NO_RELEASE_NOTES; I cannot set labels on this repository.

🤖 Generated with Claude Code

[github-actions]

✅ Release-note check exempted

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[T-Gro]

🤖 🕵️ LGTM