Handle unparsed worker profiling flag

[ShreyanshGoyal]

Fixes #1355.

Summary

• avoid reading the worker-profiling Abseil flag before flags have been parsed
• use the flag's default value in that state, consistent with Grain's config handling
• cover the JAX-enabled, unparsed-flags path with a regression test

Rationale

When JAX enables subprocess profiling support, is_worker_profiling_enabled() reaches FlagHolder.value. Abseil raises UnparsedFlagAccessError there for ordinary scripts that do not use absl.app. Falling back to the declared default preserves existing behavior until flags are parsed while leaving parsed flag values unchanged.

Test plan

• Run the issue reproduction without flags.FLAGS.mark_as_parsed() and consume a batch successfully
• Run profiler_test.py with JAX enabled (9 tests passed)
• Revert only the source fix and confirm the new regression test fails with UnparsedFlagAccessError

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📚 Documentation preview 📚: https://google-grain--1361.org.readthedocs.build/

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