Port gRPC load-balancing config parsing

Tracks copied Bun corpus position 3976 from #66: js/third_party/grpc-js/test-confg-parsing.test.ts.

Acceptance criteria:

• expose experimental.parseLoadBalancingConfig
• parse and serialize pick_first, round_robin, and outlier_detection policies
• apply upstream defaults for durations and success-rate/failure-percentage ejection settings
• preserve explicit values and undefined optional sections while remaining parse/serialize idempotent
• validate malformed/unsupported policies with cause-preserving policy-path stack context
• lower the TypeScript alias/type declarations, pass all seven upstream checks, and pin the file permanently

Parent: #66

Completed in 2bb30b3a5ac98859db7d93df3435a5d796932a5e and pushed to main.

• Added experimental.parseLoadBalancingConfig with reusable parsed config objects and lossless toJsonObject() cloning.
• Implemented pick_first, round_robin, and outlier_detection parsing, upstream duration/ejection defaults, explicit-value preservation, nested child-policy validation, and parse/serialize idempotency.
• Malformed roots, durations, percentages, ejection sections, child policies, and unsupported policies produce cause-preserving policy/path/corpus stack context.
• Focused result: 7 passed, 0 todo, 0 failed, 0 unsupported.
• The dedicated diagnostic, SQL contracts, and complete permanent third-party suite pass.