feat(firestore-bigquery-export): add the sync queue params

[cabljac]

Third layer of the Cloud Tasks write buffer stack. Adds MAX_DISPATCHES_PER_SECOND and MAX_ENQUEUE_ATTEMPTS with the extension's defaults of 100 and 3 and its validation ranges, exposes the first as a config expression for the queue's rate limit, and adds the matching optional ExportConfig fields with defaults applied in resolveExportConfig. Nothing reads the new fields yet.

One subtlety worth a look: IntParam.value() is parseInt(env || "0", 10) || 0 and never consults the declared default, so an unset param would reach resolveExportConfig as 0 and defeat the ?? 100 and ?? 3 fallbacks. optionalInt reports unset and blank env vars as undefined and passes an explicit 0 through as a real setting. The test mock for defineInt mirrors the real behaviour so the tests would catch a regression.

81 tests pass.

Part of #3031.

[gemini-code-assist]

Code Review

This pull request introduces two new configuration parameters to the Firestore BigQuery Export extension: MAX_DISPATCHES_PER_SECOND and MAX_ENQUEUE_ATTEMPTS, along with corresponding tests and helper functions to handle their defaults. The review comments correctly point out that the validation regexes for both parameters do not allow empty strings, which would cause deployment-time validation failures if left blank. Modifying these regexes to allow empty values is recommended.

[IzaakGough]

Approving. Params, defaults and ranges match the extension, and optionalInt is guarding a real case: prepare.ts only copies a resolved param into the function env if (envValue && ...), so a blank is dropped and the var really is absent at runtime. tsc --noEmit is clean and the suite passes.

One non-blocking note on the two regex comments, inline. There is no runtime difference either way, so this can land as-is and be tidied in the PR that wires up rateLimits.