test: push Api line coverage from 92.8% to 93.6%

[Theauxm]

Summary

Closes the easier remaining gaps in the audit pipeline and JWT builder:

• `JwtBuilder.CustomizeBearerOptions` — happy path stores the customizer, null-arg throws `ArgumentNullException`.
• `TraxAuditWriter` — three new scenarios: sink that always throws (drop-after-MaxRetries path), stop during retry backoff (cancellation propagation), quiet channel (empty-batch waiting path).
• `JsonElementConverter.ToObject` — single test exercises every `JsonValueKind` arm (object, array, string, int, double, long, true, false, null).
• `TraxAuditDisclaimerHostedService.StartAsync` / `StopAsync` (via reflection since the type is internal).

The remaining big chunks (OIDC pipeline, GraphQL TypeModule field building) need full TestServer scaffolding to hit and are out of scope for this PR.

Test plan

• `dotnet test tests/Trax.Api.Tests` — passing (4 new in CoverageEdgeTests, 3 new in TraxAuditWriterTests)
• `dotnet csharpier check .` — clean

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[traxsharp]

This PR is included in version 1.25.0