Fix: remove the dead AgentLoader.listApps() and getAppFile() pair

[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: AgentLoader in dev/src/utils/agent_loader.ts carries two methods that nothing in dev/src calls: listApps() and getAppFile(). getAppFile() only forwards to getAgentFile(). listApps() loads every discovered candidate a second time through agentFile.load() only to run an isApp() test, which makes the dead method the most expensive one on the class.

Solution: I chose option (a) and deleted both methods. Four facts support that choice: no caller exists in dev/src; AgentLoader is not exported from dev/src/index.ts, which exports only AdkApiClient and AdkApiServer; getAppFile() was a pure alias for getAgentFile(); and the Python reference AgentLoader has no list_apps(), while its list_agents_detailed() flattens an App to its root agent instead of reporting an App/agent distinction. I rejected option (b), wiring up a consumer, because it needs a new GET /list-apps response shape and a UI consumer that neither this repository nor the Python reference asks for.

Notes for the reviewer:

• GET /list-apps is unchanged. The route at dev/src/server/adk_api_server.ts:244 calls listAgents(), and this change does not edit listAgents() or getAgentFile().
• AdkApiClient.listApps() in dev/src/server/adk_api_client.ts is a different class with a live caller. I did not touch it.
• Two existing tests were edited, not added. The normal rule is to add a test. The exception applies here because both tests call the API being removed. Each one keeps its live assertions:
  • dev/test/utils/agent_loader_test.ts still asserts that a directory whose entrypoint is app.js is discovered, and that the loaded export is an App named test_app. Only the listApps() / getAppFile() calls changed, to listAgents() / getAgentFile().
  • tests/integration/app_loader/app_loader_test.ts keeps all four listAgents() assertions verbatim and now classifies each discovered name through getAgentFile() + isApp(). The new expect(appNames.sort()).toEqual(['service_alpha', 'standalone_app']) is stronger than the toHaveLength(2) plus two toContain assertions it replaces. The other two cases changed getAppFile to getAgentFile and nothing else.
• This change is independent of the in-flight lazy-discovery refactor on feat/lazy-agent-discovery (Feat: split agent discovery from agent loading in the dev AgentLoader #633), which left both methods alone. This branch is cut from main, not stacked on it.
• Collision check: I listed all 623 open pull requests on the fork and searched them for listApps and getAppFile. No pull request removes either method. The pull requests that mention them (Feat: split agent discovery from agent loading in the dev AgentLoader #633, Fix: bound AgentLoader bundle imports so app_loader discovery test stops timing out on Windows/macOS CI #247, Perf: scope the agent-loader esbuild plugin filter and skip node_modules during discovery #264, Test: drop the npm install from the AgentLoader discovery integration fixture #276, Fix: stop the app_loader discovery test billing its fixture setup to the first it() (Part 1/2) #506) only touch the same test files or add tests that call listApps(). None of them is a duplicate of this change.
• AgentFile.loadApp() and AgentFile.loadAgent() on the sibling class are also caller-free in dev/src. loadApp() has real App-synthesis behaviour, so removing it is a behaviour change and not a deletion. It is filed as a separate task and is out of scope here.

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/utils/agent_loader_test.ts        # 31 passed
npx vitest run --project unit:dev dev/test/server/adk_api_client_test.ts     # 20 passed
npx vitest run --project integration tests/integration/app_loader/app_loader_test.ts   # 6 passed
npm run lint          # clean
npm run format:check  # clean

npm run ts:check reports 280 errors. All of them are pre-existing in core/test/**; the error set is byte-identical before and after this change, and none of them names a file in this diff.

Mutation tests. Both rewritten assertions were run against mutated source and both failed.

1. Unit test. I deleted the app.* entrypoint preference in loadAgentFromDirectory(), so only agent.* is discovered:

   -      subFiles.find((f) => f.isFile && f.name === 'app' && isJsFile(f.ext)) ??
          subFiles.find((f) => f.isFile && f.name === 'agent' && isJsFile(f.ext));

   discovers app entrypoint files (e.g. app.js) in directories FAILED:
   AssertionError: expected [ 'agent1', 'agent2', 'agent3' ] to include 'my_service'.

2. Integration test. I stopped AgentFile.load() caching the App for a module that exports app, so it returns the root agent instead:

        if (isApp(jsModule.app)) {
   -      this.app = jsModule.app;
          this.agent = jsModule.app.rootAgent;
   -      return this.app!;
   +      return this.agent!;
        }

   should discover apps vs agents across directories and standalone files FAILED:
   expected [] to deeply equal [ 'service_alpha', 'standalone_app' ].
   This mutation also fails the pre-existing should load App from directory entrypoint case, so the new assertion is not the only detector, but it does carry signal.

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 web tests/integration/app_loader/discovery --port 8477
curl -s localhost:8477/list-apps

Result: ["service_alpha","service_beta","standalone_agent","standalone_app"], the expected sorted array.

One environment note: the integration suite runs npm install in its fixture directory, and that install fails on my machine against a private registry mirror. I ran it with npm_config_registry=https://registry.npmjs.org. The failure is local and unrelated to this change, so I did not weaken the test.

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.