fix: bound runner shutdown waits

[intel352]

Summary

• Bound post-cancel waits in the GitHub runner provider so terminal GitHub job success can return proof even if runner shutdown lags.
• Apply the same bounded wait to dispatch-failure and timeout paths.
• Add regressions for terminal success and timeout when the runner process never closes its done channel.

Verification

• GOWORK=off go test ./cmd/github-actions-runner-job -run 'TestT915WaitForGitHubCompletionDoesNotBlockOnRunnerShutdownAfterTerminalSuccess|TestT915CommandTreatsGitHubJobAPIAsCompletion|TestT915CommandTreatsRunnerSuccessMarkerAsCompletion|TestT915CommandRejectsFailedGitHubJobAPICompletion|TestT915CommandRejectsRunnerExitBeforeGitHubJobTerminal|TestT915CommandAllowsBriefGitHubJobAPICompletionLagAfterRunnerExit' -count=1 PASS
• GOWORK=off go test ./cmd/github-actions-runner-job -run 'TestT915WaitForGitHubCompletionDoesNotBlockOnRunnerShutdownAfterTerminalSuccess|TestT915WaitForGitHubCompletionDoesNotBlockOnRunnerShutdownAfterTimeout|TestT915CommandTreatsGitHubJobAPIAsCompletion|TestT915CommandTreatsRunnerSuccessMarkerAsCompletion|TestT915CommandRejectsFailedGitHubJobAPICompletion|TestT915CommandRejectsRunnerExitBeforeGitHubJobTerminal|TestT915CommandAllowsBriefGitHubJobAPICompletionLagAfterRunnerExit' -count=1 PASS
• GOWORK=off go test ./cmd/github-actions-runner-job -count=1 PASS
• GOWORK=off go test ./internal -run 'EphemeralRunner|RunnerProvider|GitHubRunner' -count=1 PASS
• GOWORK=off go test ./... -count=1 PASS

Revert Proof

With terminal-success bounded wait reverted:

• GOWORK=off go test ./cmd/github-actions-runner-job -run TestT915WaitForGitHubCompletionDoesNotBlockOnRunnerShutdownAfterTerminalSuccess -count=1 FAIL: waitForGitHubCompletion blocked on runner shutdown after terminal success

With timeout bounded wait reverted:

• GOWORK=off go test ./cmd/github-actions-runner-job -run TestT915WaitForGitHubCompletionDoesNotBlockOnRunnerShutdownAfterTimeout -count=1 FAIL: waitForGitHubCompletion blocked on runner shutdown after timeout

With fixes restored:

• focused tests and full ./... suite pass.

STG Evidence Context

• STG workload task github-provider-dogfood-linux-20260630182045 reached leased with zero proofs while target workflow 28466630619 completed successfully on the ephemeral provider runner. This PR addresses the provider finalization hang that prevents returning github-runner-proof.json to workflow-compute.