feat: Add neutral job conclusion via allow-failure property

[atkinsonm]

Summary

• Adds TaskResult.Neutral (value 6) and ActionResult.Neutral (value 4) enum values
• Adds AllowFailure boolean property to AgentJobRequestMessage
• When a job fails and allow-failure is set, the runner reports Neutral instead of Failed
• Includes ACTIONS_ALLOW_FAILURE env var fallback for testing without server changes
• Adds unit tests covering merge behavior, return code translation, and the override logic

Why

The Checks API supports a neutral conclusion (renders as a grey dash icon), but GitHub Actions provides no way for a job to report it. This means non-critical checks (linting, spell-checking, advisory security scans) must either pass or fail — there's no "warning" state.

This PR implements the runner-side support for a neutral conclusion via a job-level allow-failure property, similar in spirit to Travis CI's allow_failures matrix config. When the server populates AllowFailure = true on the job message, the runner converts Failed → Neutral at job completion time.

Related: #2347

Server-side requirements (out of scope for this PR)

For end-to-end functionality, the server needs to:

1. Parse allow-failure: true from workflow YAML
2. Set AllowFailure = true on AgentJobRequestMessage
3. Accept TaskResult.Neutral (value 6) and map it to Checks API conclusion: neutral
4. Treat neutral as non-blocking for downstream needs: dependencies

Design decisions

• Override location: CompleteJobAsync() after jobContext.Complete() — same pattern as other post-completion transforms
• Neutral is terminal: MergeTaskResults() already handles this correctly since Neutral = 6 > Failed = 2, so the existing if (currentResult > TaskResult.Failed) guard treats it as terminal
• Env var fallback: ACTIONS_ALLOW_FAILURE=true allows testing the runner behavior in isolation without server changes

Test plan

• Unit tests added for:
  • AllowFailure message property converts Failed → Neutral
  • AllowFailure does not affect Succeeded jobs
  • ACTIONS_ALLOW_FAILURE env var fallback works
  • Both CompleteJobAsync overloads (IRunServer + IJobServer) are covered
  • TaskResult.Neutral round-trips through return code translation
  • TaskResult.Neutral maps to ActionResult.Neutral
  • Neutral is terminal in merge logic (not overwritten by subsequent results)
• End-to-end testing requires server-side changes (env var fallback demonstrates runner behavior in isolation)

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