ci(stryker): re-enable pull_request trigger once Buildalyzer supports Microsoft.CodeAnalysis 5.6+

[Chris-Wolfgang]

Context

PR #269 originally added a pull_request trigger to .github/workflows/stryker.yaml per the #173 gate design. In practice, dotnet-stryker 4.15 and 4.16 both crash at initialisation on this repo:

```
System.TypeInitializationException: The type initializer for
'Microsoft.CodeAnalysis.VisualBasic.VisualBasicCommandLineParser'
threw an exception.
---> System.TypeLoadException: Could not load type
'Roslyn.Utilities.IObjectWritable' from Microsoft.CodeAnalysis,
Version=5.6.0.0
```

Root cause: Stryker's bundled Buildalyzer targets an older Microsoft.CodeAnalysis than the 5.6 dragged in transitively by the fleet analyzer bumps in v0.3.5:

• Microsoft.CodeAnalysis.BannedApiAnalyzers 4.14.0 → 5.6.0 (Bump Microsoft.CodeAnalysis.BannedApiAnalyzers from 4.14.0 to 5.6.0 #240)
• Microsoft.CodeAnalysis.PublicApiAnalyzers 3.3.4 → 5.6.0 (Bump Microsoft.CodeAnalysis.PublicApiAnalyzers from 3.3.4 to 5.6.0 #241)
• SonarAnalyzer.CSharp 10.27 → 10.29 (transitive MC pull)

Current state (as of this issue)

stryker.yaml runs on:

• workflow_dispatch — manual
• schedule — weekly Sunday 06:00 UTC

pull_request trigger is disabled with a comment referencing this issue.

Weekly + dispatch triggers ALSO fail with the same crash today, so the break: 60 threshold in stryker-config.json is effectively an aspirational floor until this is fixed. When you dispatch the workflow, it will crash — that's how you'll know when this issue can close.

Acceptance criteria

• A dotnet-stryker release that either bundles Buildalyzer 8.x+ (which supports MC 5.6+) OR ships its own Buildalyzer-alternative works against this repo's dependency graph without the IObjectWritable TypeLoadException.
• stryker.yaml re-adds the pull_request trigger against main + vNext (paths-filtered to src/, tests/, config, workflow).
• A dispatch run completes and reports a mutation score against the break: 60 threshold in stryker-config.json.

References

• Crash: PR #269 run 29527854349 — same on both 4.15.0 and 4.16.0.
• Stryker upstream: https://github.com/stryker-mutator/stryker-net (open issues on Roslyn 5.x compatibility likely tracked there).
• MC 5.6 came from PRs Bump Microsoft.CodeAnalysis.BannedApiAnalyzers from 4.14.0 to 5.6.0 #240 and Bump Microsoft.CodeAnalysis.PublicApiAnalyzers from 3.3.4 to 5.6.0 #241 in v0.3.5.