fix: deliver cancel ack and reap leftover Windows job processes (#996)

## Summary

- When Gitea cancels a job, the reporter cancels its own task context; the final Close() flush then aborted on that same cancelled context and Gitea never received the runner's acknowledgement (missing tail logs and final state).
- On Windows the cancelled context also neutralised terminateRunningProcesses, leaving step grandchildren alive in the workspace, holding file handles, so the runner could no longer clean up and pick up new work.
- Reporter.Close() now flushes on a detached, bounded context via a new rpcCtx() helper and configurable Runner.ReportCloseTimeout (default 10s).
- terminateRunningProcesses now PowerShell-enumerates Win32_Process and taskkill /T /F's every process whose ExecutablePath or CommandLine references the job's workspace directories, on a detached context.
- The daemon heartbeat loop still exits on <-r.ctx.Done(): the runner is intentionally seen as offline by Gitea during cleanup so it isn't handed a new task overlapping the in-progress teardown.

## Test plan

- [x] go test ./internal/pkg/report/... ./act/container/ -run 'TestReporter_ServerCancelStillFlushesFinal|TestBuildWindowsWorkspaceKillScript'
- [x] make fmt && make lint-go - 0 issues
- [x] GOOS=windows go build ./... - clean
- [x] Manual on a Windows runner: trigger a long-running workflow, cancel from Gitea UI; verify (a) the job ends with tail logs + cancelled state in Gitea, (b) workspace cleans up, (c) the runner picks up a new job without restart.

Authored-by: bircni
🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/996
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>

internal/pkg/config/config.example.yaml

@@ -60,6 +60,9 @@ runner:
   # The interval for reporting task state (step status, timing) to the Gitea instance.
   # State is also reported immediately on step transitions (start/stop).
   state_report_interval: 5s
+  # Per-attempt deadline for flushing the final logs and task state when a job
+  # finishes, on a detached context so a server cancel can't block the acknowledgement.
+  report_close_timeout: 10s
   # The github_mirror of a runner is used to specify the mirror address of the github that pulls the action repository.
   # It works when something like `uses: actions/checkout@v4` is used and DEFAULT_ACTIONS_URL is set to github,
   # and github_mirror is not empty. In this case,