feat: export AdkApiServer from @google/adk-devtools package

[kalenkevich]

Link to Issue or Description of Change

Related to: #238

Problem:
The @google/adk-devtools package did not export AdkApiServer or AdkApiClient for programmatic use, limiting its functionality to CLI usage only. Additionally, the package lacked proper support for dual CommonJS and ESM exports.

Solution:

• Created dev/src/index.ts to act as the main entry point for programmatic exports, exposing AdkApiServer and AdkApiClient.
• Updated dev/package.json to support dual exports (ESM and CommonJS) via the "exports" field and added "typings".
• Updated dev/tsconfig.json to output types to dist/types.
• Added integration tests in tests/integration/build_setup to verify that AdkApiServer can be successfully imported and used in various environments (Node.js CJS/ESM, TypeScript CJS/ESM).

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:
Added should import devtools successfully and should run devtools CLI successfully tests to build_setup_test.ts.

Manual End-to-End (E2E) Tests:

• Verified via automated build setup tests running across multiple configurations (JS/TS, ESM/CJS).

Checklist

• I have performed a self-review of my own code.
• 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.

[wbyoung]

Thanks for the quick turnaround.