Fix a thread-safety issue in Runner.Worker that leads to job crashes

[sebastiandero]

When introducing "parallel:" into a job, we hit a thread-safety issue in the runner code.
This PR adds a regression test and fixes the problem. We have tested this in production on our side to fix the issue and the regression test is red/green tested.

Error signature (many actions can hit this problem)

SyntaxError: Unexpected end of JSON input
    at JSON.parse (<anonymous>)
    at new Context (.../buf-action/<sha>/dist/index.js:1871:37)
    ...
    at Module._compile (node:internal/modules/cjs/loader:1854:14)
Node.js ...

I did not find a "contributing" doc or guide so please excuse any mistakes in this submission, let me know what to change.

Thank you!
Seb

[Copilot]

Pull request overview

This PR addresses a race in Runner.Worker.ExecutionContext.WriteWebhookPayload() where concurrent readers of event.json (e.g., in parallel: steps) can observe a partially-written file and crash when parsing JSON. It implements an atomic write pattern (write to temp file + replace) and adds an L0 regression test that stresses concurrent rewrites/readers.

Changes:

• Update ExecutionContext.WriteWebhookPayload() to write the GitHub event payload via a temp file and atomically replace event.json.
• Add an L0 regression test that repeatedly rewrites the payload while a concurrent reader verifies it never observes truncation.
• Add supporting test imports for file IO and task-based concurrency.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/Runner.Worker/ExecutionContext.cs	Switch webhook payload persistence to a temp-write + atomic replace approach to prevent truncated reads under concurrency.
src/Test/L0/Worker/ExecutionContextL0.cs	Add a concurrency regression test covering repeated rewrites of event.json while a parallel reader validates the content is never truncated.