Merge Queue: Isolate PRs from Batching with noBatch

PRs can now opt out of batching to test in isolation. When a high-risk PR is in the queue, batching it with other PRs means a failure forces the entire batch to restart.

PRs can now opt out of batching to test in isolation. When a high-risk PR is in the queue, batching it with other PRs means a failure forces the entire batch to restart. With the new noBatch flag, a PR tests on its own, protecting other PRs from being delayed by its potential failures. Set it via the /trunk merge --no-batch CLI command or by passing "noBatch": true in the POST /v1/submitPullRequest API request.