Honor JSON byte budgets for audit SARIF

[Widthdom]

Summary

• preflight complete recipe/audit SARIF against the exact UTF-8 stdout budget
• omit only whole results with safe recovery metadata and no partial SARIF
• document and test exact boundaries, Unicode, oversized/empty/multi-query runs, help, and completion

Root cause

Audit/recipe SARIF streamed directly to stdout and the generic byte-limit validation rejected SARIF, so no complete-document preflight or safe truncation contract existed.

Validation

• dotnet build CodeIndex.sln -c Release -m:1 -p:UseSharedCompilation=false --no-restore
• targeted Issue Add a byte budget to audit SARIF output #4903 / Add SARIF output for audit recipe results #4715 tests: 8 passed on net8.0 and 8 passed on net9.0
• SARIF / Issue Add SARIF output for audit recipe results #4715 regression tests: 11 passed on net8.0 and 11 passed on net9.0
• documentation contract tests: 14 passed on net8.0 and 14 passed on net9.0
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• changelog fragment check: 83 fragments validated
• two-round Codex adversarial review; all findings addressed
• final scoped cdidx index refresh and status --check --json: fresh, complete, and no failed checks; an earlier unscoped retry exposed the unrelated FTS cleanup failure tracked by Full index can fail while restoring FTS triggers after graph finalization #5012
• full suite attempted; the known GitHelper full-suite timeout is tracked by Stabilize GitHelper timeout regression test under full-suite load #4982, and two load-sensitive cases passed isolated reruns before the long suite was stopped during I/O wait

Documentation and changelog

• updated README.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md in English and Japanese
• added changelog.d/unreleased/4903.fixed.md

Related: #4982
Related: #5012

Fixes #4903