Stabilize UTF-8 symbol worker test startup (#4937)

[Widthdom]

Summary

• warm the reusable isolated symbol worker with a dedicated 30-second test-only startup budget
• keep all six Unicode protocol requests on the existing five-second callback budget, with distinct startup and callback timeout diagnostics
• document the test contract in English and Japanese and add the required changelog fragment

Root cause

The first UTF-8 request spent its fixed five-second callback budget on net9 worker process startup under parallel full-suite load. Normal process-startup contention was therefore indistinguishable from a callback hang.

Scope and impact

This is test-only. The production symbol-worker timeout contract is unchanged.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• focused net8.0/net9.0 UTF-8 and legacy-environment worker tests passed
• ten repeated parallel-TFM runs of the UTF-8 regression test passed on each TFM
• the exact full-solution Release test command passed the target test on both TFMs; the overall run was not green because the unrelated test tracked by Stabilize parallel batch snapshot refresh timeout in the net9 full suite #5015 timed out on both TFMs and the performance smoke budget was exceeded once on net8.0 under simultaneous multi-checkout load; focused reruns passed
• documentation contract tests passed
• dotnet run --project tools/CodeIndex.Changelog -- check
• Codex adversarial review: no actionable findings

Follow-up

• Stabilize parallel batch snapshot refresh timeout in the net9 full suite #5015 tracks the unrelated parallel batch snapshot timeout seen during full-suite validation

Fixes #4937