Reconcile external symlink retargets in watch mode

[Widthdom]

Summary

• Preserve watch reconciliation for an indexed in-workspace symlink when --follow-symlinks internal later resolves it outside the workspace.
• Keep the lexical link path as the cleanup input without opening or indexing the disallowed external target.
• Add deterministic end-to-end regression coverage for stale file and symbol removal.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false (0 warnings, 0 errors)
• Issue Reconcile indexed symlinks retargeted outside the workspace in watch mode #5120 focused regression on Release net8.0 and net9.0 (passed)
• IndexWatchRunnerTests on Debug net8.0 (78 passed) and the new regression on Debug net9.0 (passed)
• Full solution tests on Release net8.0/net9.0 (23,017 passed, 430 skipped, 0 failed)
• dotnet run --project tools/CodeIndex.Changelog -- check (38 fragments validated)
• Root and workspace index checks passed after a full refresh and fold backfill

Documentation and changelog

• Updated the English and Japanese sections of DEVELOPER_GUIDE.md and TESTING_GUIDE.md.
• Added changelog.d/unreleased/5120.fixed.md.

Adversarial review

No blocking/actionable issues found.

Follow-up candidates

None.

Fixes #5120