fix: prevent duplicated UpdateFunction calls

[IzaakGough]

Fixes #10731.

• Per-request retryOnPrematureClose flag. The API client resends a request once without keep-alive after a premature close. That error is raised while reading the response, after the server has already accepted the request, so resending a write can repeat it. The new flag (default true) lets a caller opt out.
• Disabled for Cloud Functions writes. Create, update and delete calls for v1 and v2 set the flag to false. A resent UpdateFunction fails with 409 ABORTED, a resent create would hit the function the first call just made, and a resent delete would target one already being removed.

Unit tests in src/apiv2.spec.ts cover the retry with the flag off and on. The premature close was not reproduced against the live API.

[gemini-code-assist]

Code Review

This pull request restricts retrying requests on premature close errors to specific HTTP methods by introducing a helper function, and adds corresponding unit tests. The review feedback correctly notes that the helper function is named isIdempotentMethod but includes POST, which is non-idempotent. It suggests renaming the function to shouldRetryMethod to maintain semantic correctness.