fix: make App Hosting emulator startup and configuration errors non-blocking

[christhompsongoogle]

Description

Prevent missing configurations and startup failures for the App Hosting emulator from crashing emulators:start and blocking the rest of the emulator suite:

• Check for package manager lockfiles (package-lock.json, yarn.lock, pnpm-lock.yaml) in shouldStart when startCommand is unconfigured, skipping startup with an error log if none are found.
• Gracefully handle App Hosting emulator startup failure in startAll by cleaning up the failed instance and logging the error without crashing other emulators.

Fixes b/546204399

Scenarios Tested

• Unit tests in controller.spec.ts for App Hosting shouldStart handling (missing lockfile, startCommand configured, and lockfile present).

Sample Commands

• firebase init emulators (select all emulators)
• firebase emulators:start

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[gemini-code-assist]

Code Review

This pull request introduces graceful handling of App Hosting emulator startup failures and auto-detection of start commands from lockfiles (such as package-lock.json, yarn.lock, or pnpm-lock.yaml) when no explicit start command is configured. This prevents startup failures from blocking other emulators. The review feedback suggests improving the unit tests by configuring the fs.existsSync stub to fall back to its original implementation for non-matching arguments to avoid potential side effects during test execution.

[gemini-code-assist]

Stubbing a global/module-level method like fs.existsSync without a fallback can cause unexpected side effects or break other unstubbed calls during test execution. It is safer to configure the stub to fall back to the original implementation for non-matching arguments.

      const existsStub = sinon.stub(fs, "existsSync");
      existsStub.callsFake(existsStub.wrappedMethod);
      existsStub.withArgs(sinon.match(/package-lock\.json/)).returns(true);

[wiz-9635d3485b]

Wiz Scan Summary

Scanner	Findings
Vulnerabilities	-
Sensitive Data	-
Secrets	-
IaC Misconfigurations	-
SAST Findings	5  3
Software Management Findings	-
Total	5  3

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