fix(serve): fit provider confirmation answers to the response limit #13038

What happened?

The Managed Runtime provider worker fits execute, status and cancel results into their per-kind wire budget with fitManagedRuntimeProviderResult, which #12868 added. It does not fit confirmation answers: they are validated and sent as they are, under the same 1 MiB limit.

For a Session that is not preapproved, a write_file or edit over a file of about 900 KB produces a confirmation larger than that limit. The confirmation carries the diff and the original and new contents. The worker route then answers 400 managed_runtime_provider_invalid at confirmation.

The real-stack verification of #12868 measured this (round 6, section 4):

This predates #12868. That PR kept it out of scope and recorded it as follow-up in its update comment.

What did you expect to happen?

The Harness should get a confirmation it can show for a large edit. Either:

• cut the bulk fields (fileDiff, originalContent, newContent) with a notice, the way results are fitted, keeping the fields that each variant requires; or
• answer with a definitive, typed refusal that the caller can act on.

Either way, it should not be a protocol-invalid 400 that reads like a malformed answer.

Client information

Not client-specific. It reproduces with the bundled Managed Runtime worker (managed-runtime-provider/1) on main at 1f1209bc70.

Anything else we need to know?

• The fitter is fitManagedRuntimeProviderResult in packages/cli/src/serve/managed-runtime-provider-protocol.ts. confirmation is outside its execute/status/cancel set.
• Core's serializeManagedToolConfirmation emits each variant's required fields. feat(serve): implement generic Broker provider controls #12868 made the client require them (edit: fileName, filePath, fileDiff, newContent, and originalContent as a string or null). Any fitting has to keep those fields present.

中文：provider worker 只对 execute/status/cancel 的结果做裁剪，confirmation 不裁剪。未预先批准的 Session 对约 900 KB 的文件做 write_file/edit 时，confirmation 会超出 1 MiB 上限，返回 400 managed_runtime_provider_invalid。这个问题早于 #12868。期望的做法是按变体保留必填字段并裁剪大字段，或者返回一个调用方能处理的明确拒绝。