perf: avoid activity id conversion allocations

[Sean-Kenneth-Doherty]

Fixes #4289.

#skip-changelog

Summary

• Add internal span/byte formatting helpers for SpanId and SentryId.
• Convert Sentry IDs to OpenTelemetry Activity*Id values with stackallocated byte buffers on net8.0+, avoiding the intermediate ToString() allocation.
• Keep stackallocated char-buffer fallbacks for legacy target frameworks.
• Add explicit fixed-value conversion coverage for high-bit span IDs and trace IDs.

Verification

• dotnet test /home/sean/.cache/codex/oss-contrib/sentry-dotnet-otel-alloc/test/Sentry.OpenTelemetry.Exporter.Tests/Sentry.OpenTelemetry.Exporter.Tests.csproj --framework net10.0 --no-restore /p:MSBuildEnableWorkloadResolver=false /p:WorkloadList=none /p:NO_MOBILE=true
• dotnet build /home/sean/.cache/codex/oss-contrib/sentry-dotnet-otel-alloc/src/Sentry.OpenTelemetry.Exporter/Sentry.OpenTelemetry.Exporter.csproj --framework netstandard2.0 --no-restore /p:MSBuildEnableWorkloadResolver=false /p:WorkloadList=none /p:NO_MOBILE=true
• dotnet build /home/sean/.cache/codex/oss-contrib/sentry-dotnet-otel-alloc/src/Sentry.OpenTelemetry.Exporter/Sentry.OpenTelemetry.Exporter.csproj --framework net462 --no-restore /p:MSBuildEnableWorkloadResolver=false /p:WorkloadList=none /p:NO_MOBILE=true
• git diff --check

AI assistance: This PR was prepared with OpenAI Codex.

[Sean-Kenneth-Doherty]

Local validation on 413dd80b using the repo-requested .NET SDK 10.0.203 from global.json:

• dotnet test test/Sentry.OpenTelemetry.Exporter.Tests/Sentry.OpenTelemetry.Exporter.Tests.csproj --framework net10.0 --no-restore /p:MSBuildEnableWorkloadResolver=false /p:WorkloadList=none /p:NO_MOBILE=true -> 59 passed
• dotnet build src/Sentry.OpenTelemetry.Exporter/Sentry.OpenTelemetry.Exporter.csproj --framework netstandard2.0 --no-restore /p:MSBuildEnableWorkloadResolver=false /p:WorkloadList=none /p:NO_MOBILE=true -> passed
• dotnet build src/Sentry.OpenTelemetry.Exporter/Sentry.OpenTelemetry.Exporter.csproj --framework net462 --no-restore /p:MSBuildEnableWorkloadResolver=false /p:WorkloadList=none /p:NO_MOBILE=true -> passed
• git diff --check origin/main...HEAD and git diff --check -> clean

The netstandard2.0 and net462 builds emit the existing UnoptimizedAssemblyDetector warning for Sentry.dll, but complete with 0 errors.

[jamescrosswell]

Thanks @Sean-Kenneth-Doherty .

I added a couple of commits to tidy up the code from Codex (the code was technically correct but it wasn't very obvious what that code was doing, so hard to maintain).

[codecov]

Codecov Report

❌ Patch coverage is 77.27273% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.10%. Comparing base (a9cb3b2) to head (e7d3326).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Sentry/SentryId.cs	45.45%	2 Missing and 4 partials ⚠️
src/Sentry/SpanId.cs	78.94%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5239      +/-   ##
==========================================
- Coverage   74.12%   74.10%   -0.02%     
==========================================
  Files         508      508              
  Lines       18282    18315      +33     
  Branches     3574     3581       +7     
==========================================
+ Hits        13551    13573      +22     
- Misses       3861     3870       +9     
- Partials      870      872       +2     

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