Feature Request: CLI/API to get App Testing Agent test run status and results

## Feature Request

### Description
The App Testing Agent (AI-powered testing) for Firebase App Distribution currently has no CLI command or REST API to programmatically retrieve test run status and results. This prevents integration into CI/CD pipelines for automated quality gates.

### Current State
- `firebase appdistribution:testcases:import` - ✅ Works for importing test cases
- `firebase appdistribution:distribute --test-devices=... --test-case-ids=...` - ✅ Works for triggering tests
- **Getting test results** - ❌ Only available via Firebase Console UI

### Gradle Plugin Investigation

We investigated the Gradle plugin as a potential workaround. The `firebaseAppDistribution` block supports a `testNonBlocking` parameter:

```kotlin
firebaseAppDistribution {
testDevices = "model=shiba,version=34,locale=en,orientation=portrait"
testCases = "prescriptive-workouts"
testNonBlocking = true // or false?
}
```

**Documentation gap:** The [Gradle distribution docs](https://firebase.google.com/docs/app-distribution/android/distribute-gradle) only describe `testNonBlocking = true`:

> "Run automated tests asynchronously. Visit the Firebase console for the automatic test results."

**What's NOT documented:**
- What happens when `testNonBlocking = false` (or omitted)?
- Does the Gradle task block until tests complete?
- Are exit codes returned (0 for pass, non-zero for fail)?
- What's the timeout behavior?

If `testNonBlocking = false` provided blocking behavior with exit codes, this would be a partial solution. But the behavior is undocumented.

### Requested Feature
Add CLI commands and/or REST API endpoints to:

1. **List test runs** for a release
```bash
firebase appdistribution:testruns:list --app --release
```

2. **Get test run status/results**
```bash
firebase appdistribution:testruns:get --app
```

Should return JSON with:
- Test case ID and name
- Status (pending, running, passed, failed)
- Device information
- Timestamp
- Link to detailed results
- (Optionally) Issues found, screenshots

3. **Wait for test completion** (for CI/CD blocking)
```bash
firebase appdistribution:testruns:wait --app --release --timeout 30m
```
Exit code 0 if all pass, non-zero if any fail.

### Use Case
We're using the App Testing Agent to run E2E tests defined in YAML. We want to:
1. Build APK in CI
2. Distribute to App Testing Agent
3. **Wait for tests to complete and get results** ← Currently impossible
4. Fail the CI pipeline if tests fail

### Workaround Attempts
- Firebase App Distribution REST API (`firebaseappdistribution.googleapis.com`) - No test result endpoints
- Firebase Test Lab gcloud commands - Different service, doesn't cover App Testing Agent
- Cloud Functions triggers - App Testing Agent completion events not documented
- Gradle plugin `testNonBlocking` - Behavior undocumented (see above)

### Environment
- Firebase CLI version: 13.x
- Feature: App Testing Agent (Preview)

### Additional Context
The App Testing Agent is a powerful feature, but without programmatic result access, it can only be used for manual/ad-hoc testing rather than automated quality gates. Adding this would significantly increase adoption for teams with CI/CD pipelines.

Could the Firebase team also clarify the `testNonBlocking = false` behavior in the Gradle plugin? If it already supports blocking with exit codes, documenting this would help teams in the interim.

Start by reviewing the existing firebase appdistribution:testcases:import and firebase appdistribution:distribute commands, then compare the firebaseappdistribution Gradle documentation for testNonBlocking. The issue names no files or tests; done would require an agreed CLI/API design that lists test runs, retrieves results, and supports waiting with CI exit behavior.

Written by the indexing model from the issue text.