Fix flaky BigQuery persistent retry test

[bvolpato]

Fixes #39526.

Summary

The persistent retry test asserts one initial BigQuery call plus three retries. Its default pipeline can select Prism, where runner-dependent execution sometimes invokes the mocked retry path twice. The reported failure saw 8 calls instead of 4.

Pin this runner-specific mocking test to FnApiRunner, matching the adjacent intermittent retry test. Production code is unchanged.

Validation

• Confirmed failing CI trace contained two four-attempt retry sequences
• Unchanged test: 50/50 local passes, consistent with a rare scheduling flake
• Python 3.12 cloud tox target after fix: 2 passed
• Patched timeout case stress: 50/50 passes
• YAPF 0.43.0 and Ruff 0.15.7 passed on changed file

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[github-actions]

Assigning reviewers:

R: @shunping for label python.

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[shunping]

LGTM! Thanks!