🤖 perf: zod 4.6.5 follow-ups (Bedrock/OpenRouter live check, API chunk growth) #6091

Follow-ups from the zod 4.6.5 bump (#6078). Each one waits for its trigger.

1. Bedrock and OpenRouter accept nullable tool fields as type: [X, "null"] according to their docs only. Nobody tested it live. Trigger: a key for either provider, or a user report of a rejected tool schema there.
2. The served API-*.js chunk grew by about 7.3 KB brotli. My guess is zod 4.6.5's own code, but nobody checked. Trigger: the next first-load byte review, or PR8 (🤖 ci: fail PRs that grow first-load JS over the recorded budget #6084) restarting.
3. The 3 make test failures that the lane saw locally (taskGitPatchEngine x2, BackupRepoCache timeout) were compared with base on the pre-rebase head only. CI was green on the final head. Trigger: they reproduce on a clean host.

Refs #6078, #6071, #6080, #6087, #6083.

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