Feat: Configurable Client Labels in adk-js

[AmaadMartin]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Related: # (No specific issue link provided, parity feature with adk-python)

2. Or, if no issue exists, describe the change:

Problem:
Developers need to be able to configure custom client labels dynamically for HTTP requests made by the ADK JS library for tracking, debugging, and evaluation (parity with adk-python). We also need to extract cleaner browser info from User Agent strings instead of sending full raw UA.

Solution:

• Implemented runWithClientLabel and updated getClientLabels to support context-propagated labels using AsyncLocalStorage in Node.js.
• Provided a browser-safe shim for AsyncLocalStorage in core/src/utils/async_hooks_shim.ts.
• Configured esbuild in core/build.js to alias node:async_hooks to the shim when building for browser platform with bundle option.
• Implemented parseUserAgent in core/src/utils/client_labels.ts to extract Browser/Version from UA.
• Exported new utilities in core/src/common.ts.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Summary of passed npm test results:

 RUN  v3.2.6 /usr/local/google/home/amaadmartin/Workspace/Agentspaces/feat-configurable-client-labels/adk-js

 Test Files  2 passed (2)
      Tests  24 passed (24)
   Start at  15:45:00
   Duration  10.32s

Passed tests include:

• client_labels tests (17 tests) covering default labels, UA parsing, context propagation, and error cases for empty labels.
• BaseLlm tests (7 tests) including a new test verifying that context client label is included in tracking headers when run within runWithClientLabel.

Manual End-to-End (E2E) Tests:

• I have manually tested my changes end-to-end.
  Verified via integration tests that BaseLlm correctly retrieves and includes the context-propagated client label in HTTP headers (x-goog-api-client and user-agent) when executed within runWithClientLabel.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context