[fix] Stop local mount polling after geesefs exits

[kkkhs]

Summary

Fixes #6559.

The local geesefs startup poll now stops as soon as the foreground child exits instead of continuing through the full 30x500ms budget. The existing 15s ceiling still applies while the child stays alive.

Testing

Verified locally

• pnpm --dir services/runner exec vitest run --project unit tests/unit/sandbox-agent-mount.test.ts
• pnpm --dir services/runner run typecheck
• pnpm --dir services/runner run build:extension
• pnpm --dir services/runner run test:unit
• git diff --check

Added or updated tests

• Added a unit case for the local mount poll returning after the geesefs exit promise resolves.

QA follow-up

N/A; runner mount timing is unit-covered. I did not reproduce the SeaweedFS multipart panic locally.

Demo

N/A; runner-only change.

Checklist

• Demo shows the real app running this branch (not a mock-up or recreated UI), or is marked N/A
• Relevant tests pass locally
• Relevant linting and formatting pass locally
• I have signed the CLA, or I will sign it when the bot prompts me

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