Redact JSON bodies with a UTF-8 BOM

[itssimon]

JSON bodies beginning with a UTF-8 byte order mark failed parsing and were exported as unmasked text. Skip a single leading BOM when creating Utf8JsonReader so JSON field masking applies to these bodies.

Validation:

• make check passed.
• make test passed all 175 existing tests on .NET 10.
• A temporary synthetic reproduction through request capture, redaction, and OTLP serialization confirmed that plain and BOM-prefixed JSON both mask password and token.
• GitHub Actions passed formatting, the .NET 8/9/10 test jobs, and the CI gate.

The Codecov checks fail because the new BOM branch is not exercised by the existing suite: patch coverage is 50.00%, and project coverage is 89.79% (-0.03 percentage points).

No tests were added. Local .NET 8 and 9 runs were unavailable because those runtimes are not installed; both passed in CI.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.79%. Comparing base (a2fb31a) to head (e0b5fcd).

Files with missing lines	Patch %	Lines
src/Apitally/Export/SpanRedaction.cs	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #54      +/-   ##
==========================================
- Coverage   89.81%   89.79%   -0.03%     
==========================================
  Files          46       46              
  Lines        3506     3507       +1     
  Branches      412      413       +1     
==========================================
  Hits         3149     3149              
  Misses        268      268              
- Partials       89       90       +1     

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[cubic-dev-ai]

No issues found across 1 file

_{Turn on auto-fix | Re-trigger cubic}