Test: fail integration tests fast when the ADK CLI is not built

[AmaadMartin]

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Link to Issue or Description of Change

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

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

Problem: AdkTsApiServer.start() spawns node dev/dist/esm/cli_entrypoint.js without checking that the file exists. A contributor who runs an integration test before npm run build gets a raw ERR_MODULE_NOT_FOUND dump and then CLI exited prematurely with code 1. That message names neither the missing file nor the fix. Five suites reach this path, including tests/integration/a2a/basic/a2a_agent_test.ts and tests/integration/adk_web/webui_test.ts.

Solution: assertAdkCliBuilt() runs as the first statement of start(). It names the resolved path and tells the developer to run npm run build, which is the order CONTRIBUTING.md documents. AdkGoServer.start() already checks a prerequisite with fs.existsSync in that position, so this follows an in-repo precedent. The guard is exported at module scope so a test can call it without widening the private getAdkCliArgs.

Scope notes:

• Only tests/integration/test_api_server.ts changes. The timeout path (an entrypoint that exists but never prints its banner) is out of scope and still costs the full 60 s.
• Collision check: I listed all 572 open pull requests on the fork and read the file lists of every adjacent one. Test: fail webui_test fast when the repo has not been built (stacked on #359) #559 adds a build guard, but it is local to webui_test.ts and also covers dev/dist/browser/index.html, which only that suite needs. Fix: allocate integration test-server ports from the OS and attach child output to start-up failures #546 touches this file, but it only removes a successLogMessage line. No open pull request adds this guard, so this branch is based on main, not stacked.

Testing Plan

Unit Tests:

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

New file tests/integration/test_api_server_test.ts, 3 tests:

npx vitest run --project integration tests/integration/test_api_server_test.ts
# 3 passed, 7 ms

Proof the tests can fail. I mutated the source twice and re-ran the same file:

Mutation	Result
Deleted assertAdkCliBuilt(CLI_ENTRYPOINT_PATH); from start()	1 failed: AssertionError: promise resolved "AdkApiClient{ backendUrl: 'http://lo…' }" instead of rejecting
Inverted the guard to if (fs.existsSync(cliPath))	3 failed: AssertionError: expected [Function] to throw an error, and expected [Function] to not throw an error but 'Error: ADK CLI entrypoint not found a…' was thrown

Coverage: vitest.config.ts measures core/src, dev/src and integrations/src only, so files under tests/ are not reported. Both branches of the new if execute, and the start() call site is pinned by the third test.

Manual End-to-End (E2E) Tests:

Reproduce an unbuilt entrypoint on a built tree, then restore it:

npm run build
mv dev/dist/esm/cli_entrypoint.js /tmp/cli_entrypoint.js.bak
npx vitest run --project integration tests/integration/a2a/basic/a2a_agent_test.ts
mv /tmp/cli_entrypoint.js.bak dev/dist/esm/cli_entrypoint.js

Before this change the run ended in CLI exited prematurely with code 1. Now it fails in 4 ms of test time with:

Error: ADK CLI entrypoint not found at <repo>/dev/dist/esm/cli_entrypoint.js.
The integration tests spawn the built CLI; run `npm run build` from the
repository root first.

Behaviour on a built tree is unchanged. Both dependent suites still pass:

npx vitest run --project integration tests/integration/a2a/basic/a2a_agent_test.ts   # 1 passed
npx vitest run --project integration tests/integration/adk_web/webui_test.ts         # 2 passed
npm run lint          # clean
npm run format:check  # clean
npm run ts:check      # 281 pre-existing errors in 41 files, identical with and without this change

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.