fix(auth): honor an explicit zero GCP metadata timeout (#2481)

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request may not be merged

## Changes being requested

`gcpIDTokenProvider` computes its metadata request timeout with
`config?.timeout || 10_000`, so an explicit `timeout: 0` is silently
replaced with 10,000 ms. The Azure IMDS provider in the same file uses
`config?.timeout ?? 10_000` for the identically documented option, so
the two providers currently behave differently for the same
configuration.

This change switches the GCP provider to the nullish coalescing the file
already uses for Azure, and adds explicit-zero regression tests for both
providers in `tests/auth/subject-token-providers.test.ts` (mocked fetch,
no live network; the Azure test is a parity control pinning the
already-correct behavior).

Verified locally (Node 24.18.0): the GCP regression fails on `main` (the
single `setTimeout` call receives `10000` instead of `0`) and passes
with this change; the provider test file passes 14/14; lint, build, and
the full test suite pass apart from the two pre-existing macOS-only
failures addressed in #2480.

## Additional context & links

One-line production change; the only other `config?.timeout` default in
the file already used `??`.