fix: contain throwing onRunStarted hook in emitRunStarted so TUI PTY runs can't orphan (#6002)

[atomantic]

Summary

• Routes the onRunStarted hook call inside emitRunStarted() (server/services/runner.js) through the existing safeSettle wrapper, so a throwing hook is logged but can never reject the caller.
• emitRunStarted fires from the TUI spawn path (server/services/tuiPromptRunner.js) after the PTY is already registered in the active-run map; previously a hook throw rejected the spawn with a live PTY and no terminal settlement — the same orphaned-child shape A throwing onRunStarted hook orphans a spawned Run Prompt child before its error listener exists #5792 closed for the CLI path.
• Signature and call site unchanged; payload-flattening logic untouched.

Test plan

• New test in server/services/runner.test.js ("contains a throwing onRunStarted hook instead of rejecting the TUI spawn path (A throwing onRunStarted hook still orphans a live PTY via the unguarded emitRunStarted helper #6002)"): verified it fails against the unpatched helper (hook error propagates out of emitRunStarted) and passes with the fix (no throw, error logged via safeSettle).
• npx vitest run services/runner.test.js services/tuiPromptRunner.test.js: 139 passed.
• Full server suite (cd server && npm test): 1907 files passed / 1 skipped, 38487 tests passed / 24 skipped.

Review note

• Local reviewer (mtplx, effort low) could not run: no model configured on the Settings → Code Reviewers page. Proceeding without a local-reviewer verdict; CI review requested instead.

Closes #6002