Switch fuzz task to UTaskLocalExecutor and improve fuzzer setup robustness

[google-labs-jules[bot]]

Switch fuzz task to UTaskLocalExecutor and improve fuzzer setup robustness.

• Change fuzz task type to UTaskLocalExecutor in task_types.py to run fuzz tasks locally.
• Update _update_fuzzer in setup.py to support downloading fuzzer binaries using blobs.read_blob_to_disk when running on a trusted bot (non-uworker), removing the strict dependency on signed URLs for local execution.
• Add unit tests for _update_fuzzer in setup_test.py covering local and uworker scenarios.

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PR created automatically by Jules for task 13398410756091827490 started by @jonathanmetzman

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