Fix: publish the bound port in A2A agent card URLs

[AmaadMartin]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):
   N/A
2. Or, if no issue exists, describe the change:
   Problem: AdkApiServer treats port: 0 as "let the OS pick a free port", but initA2A() passed the raw configured port to toA2a(). With --a2a --port 0 the served agent card therefore advertised http://localhost:0/a2a/<app>/jsonrpc. A RemoteA2AAgent resolves that card and builds its client from the card's own URL, so every call after card resolution failed.

Solution: The url getter already read the bound port off server.address(). I extracted that read into a private boundPort accessor and used it in both places, so the card URLs and url share one source of truth. core/ is deliberately unchanged: toA2a() only advertises the port it is given, so the defect was entirely in the caller.

Two notes on scope. The EADDRINUSE message still reports the configured port, because nothing bound on that path and address() is null. tests/integration/ is untouched; its harness maps a requested port of 0 onto a fixed random port, so it never reached the bug.

Collision check: gh pr list --repo AmaadMartin/adk-js --state open --limit 1000 plus gh pr diff --name-only on every adjacent A2A and port PR (#646, #584, #546, #591, #314, #558). None touch dev/src/server/adk_api_server.ts. This PR updates an existing branch in place; I rebased it from a 51-commit-stale base onto current main.

Testing Plan

Please describe the tests that you ran to verify your changes. This is required for all PRs that are not small documentation or typo fixes.
Unit Tests:
[x] I have added or updated unit tests for my change.
[x] All unit tests pass locally.

npx vitest run --project unit:dev dev/test/server/adk_api_server_test.ts — 55 passed. I added two tests and changed no existing test.

Coverage of the changed lines is 100%. Both branches of the boundPort getter and the port: this.boundPort argument are absent from the uncovered-line list. The 84.22% figure reported for the whole file is pre-existing and unrelated.

Proof each new test can fail. I ran both against unfixed code:

Mutation	Test	Failure
port: this.boundPort -> port: this.port	"should advertise the bound port in the agent card when the OS assigns it"	expected 'http://localhost:0/a2a/testApp/jsonrpc' to be 'http://localhost:40863/a2a/testApp/jsonrpc'
return this.port -> return 0	"should report the configured port before the server starts"	expected 'http://localhost:0' to be 'http://localhost:8123'

I restored the source after each mutation and confirmed with git diff that none was left behind.

No-regression check: npx vitest run --project integration tests/integration/a2a/basic/a2a_agent_test.ts — 1 passed. That test drives a real RemoteA2AAgent over a non-zero port, so it proves the unchanged path.

Manual End-to-End (E2E) Tests:
Please provide instructions on how to manually test your changes, including any necessary setup or configuration.

npm run build
node dev/dist/esm/cli_entrypoint.js api_server tests/integration/a2a/basic/remote_a2a/ --a2a --port 0
# read the bound port off the startup banner, then:
curl -s http://localhost:<bound>/a2a/weather_time_agent/.well-known/agent-card.json

Before (server bound to 45359):

url: http://localhost:0/a2a/weather_time_agent/jsonrpc
additionalInterfaces: [{"url":"http://localhost:0/a2a/weather_time_agent/jsonrpc", ...},
                       {"url":"http://localhost:0/a2a/weather_time_agent/rest", ...}]

After (server bound to 37759):

url: http://localhost:37759/a2a/weather_time_agent/jsonrpc
additionalInterfaces: [{"url":"http://localhost:37759/a2a/weather_time_agent/jsonrpc", ...},
                       {"url":"http://localhost:37759/a2a/weather_time_agent/rest", ...}]

I also pointed a RemoteA2AAgent at http://localhost:<bound>/a2a/weather_time_agent/ on both builds. Before the fix the turn failed with A2ARemoteAgent remote_a2a_agent failed: TypeError: fetch failed. After the fix it returned The weather in New York is sunny with a temperature of 25 degrees Celsius (77 degrees Fahrenheit).

Checklist

[x] I have read the CONTRIBUTING.md document.
[x] I have performed a self-review of my own code.
[x] I have commented my code, particularly in hard-to-understand areas.
[x] I have added tests that prove my fix is effective or that my feature works.
[x] New and existing unit tests pass locally with my changes.