fix: Prevent malformed log_event_data JSON for non-serializable context values

[tippmar-nr]

Summary

Fixes #3641.

When a log context-data value cannot be serialized by Newtonsoft.Json - for example a self-referencing object graph such as an ASP.NET Core Endpoint placed into a logging scope - LogEventWireModelCollectionJsonConverter fell back to writing the value's raw ToString() through WriteRawValue. WriteRawValue injects text verbatim, so the value landed in the stream unquoted and unescaped:

..."context.SomeKey":Foo.Bar.SomeType ...} ...

That makes the whole log_event_data payload invalid JSON, so the collector rejects the entire batch and every log event harvested in that cycle is dropped - not just the offending line.

Change

• Use WriteRawValue only on the SerializeObject success path (which already returns valid JSON).
• In the fallback branches, write the stringified value through WriteValue, which quotes and escapes it. A value that cannot be serialized can no longer corrupt the surrounding payload.

Test coverage

• Unit tests (LogEventWireModelCollectionJsonConverterTests): a non-serializable (self-referencing) context value, and one whose ToString() also throws, both assert the payload parses as valid JSON and the value is emitted as a quoted string.
• Integration tests (ContextDataNonSerializableTests, MEL on Core latest/oldest and FW latest): log a message with a non-serializable object in the logging scope and assert every log_event_data payload is valid JSON with the value captured as a quoted string. Verified failing without the fix and passing with it.

Author Checklist

• Unit tests added
• Integration tests added
• Performance implications considered (none expected; same code path, one extra string write)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.99%. Comparing base (3a8be50) to head (17d30cf).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...erters/LogEventWireModelCollectionJsonConverter.cs	60.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3642      +/-   ##
==========================================
+ Coverage   81.84%   81.99%   +0.15%     
==========================================
  Files         509      510       +1     
  Lines       34429    34689     +260     
  Branches     4073     4130      +57     
==========================================
+ Hits        28178    28443     +265     
+ Misses       5282     5273       -9     
- Partials      969      973       +4     

Flag	Coverage Δ
Agent	82.93% <60.00%> (+0.16%)	⬆️
Profiler	72.23% <ø> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...erters/LogEventWireModelCollectionJsonConverter.cs	97.93% <60.00%> (+8.35%)	⬆️

... and 9 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.