fix(ailogic): stream errors should not be ignored

[ncooke3]

https://github.com/firebase/firebase-ios-sdk/pull/16298/changes?w=1

Description

Fixes infinite stalls and resource leaks when streaming responses.

This surfaced as a warning.

1. Swallowed Mid-Stream Errors (Stall)

• Issue: Network errors during URLSession.AsyncBytes iteration threw silently because the unstructured Task in loadRequestStream lacked a do-catch block. This caused the continuation to stall infinitely.
• Fix: Wrapped the Task logic in a top-level do-catch that properly forwards errors via continuation.finish(throwing: error).

2. Missing Cancellation (Resource Leak)

• Issue: If a consumer broke out of the stream early, the AsyncThrowingStream continuation terminated, but the internal network request silently continued downloading and parsing in the background.
• Fix: Added an onTermination handler to the continuation (continuation.onTermination = { task.cancel() }) in both GenerativeAIService.swift and GenerativeModel.swift to explicitly cancel the underlying Task and abort the network connection.

Testing

• testGenerateContentStream_failure_midStreamError_throwsError: Verifies mid-stream drops throw correctly.
• testGenerateContentStream_cancellation_resourceLeak: Verifies early exit correctly invokes URLProtocol.stopLoading().

These failures are expected and expose the two types of bugs.

From https://github.com/firebase/firebase-ios-sdk/actions/runs/27791049770/job/82239809505?pr=16298

      - NOTE  | [OSX] xcodebuild:  /Users/runner/work/firebase-ios-sdk/firebase-ios-sdk/FirebaseAI/Tests/Unit/GenerativeAIServiceTests.swift:253: error: -[FirebaseAILogic_Unit_unit.GenerativeAIServiceTests testGenerateContentStream_cancellation_resourceLeak] : Asynchronous wait failed: Exceeded timeout of 2 seconds, with unfulfilled expectations: "stopLoading should be called when task is cancelled".
      - NOTE  | [OSX] xcodebuild:  2026-06-18 21:45:41.407428+0000 xctest[12573:40480] [[FirebaseAI]] 12.16.0 - [FirebaseAI][I-VTX002003] The server responded with an error: <NSHTTPURLResponse: 0x600002cdd7a0> { URL: https://firebasevertexai.googleapis.com/v1beta/projects/test-project-id/locations/test-location/publishers/google/models/test-model:streamGenerateContent?alt=sse } { Status Code: 400, Headers {
      - NOTE  | [OSX] xcodebuild:  /Users/runner/work/firebase-ios-sdk/firebase-ios-sdk/FirebaseAI/Tests/Unit/GenerativeAIServiceTests.swift:197: error: -[FirebaseAILogic_Unit_unit.GenerativeAIServiceTests testGenerateContentStream_failure_midStreamError_badResponse_throwsError] : Asynchronous wait failed: Exceeded timeout of 4 seconds, with unfulfilled expectations: "Stream should throw URLError(.networkConnectionLost)".
      - NOTE  | [OSX] xcodebuild:  /Users/runner/work/firebase-ios-sdk/firebase-ios-sdk/FirebaseAI/Tests/Unit/GenerativeAIServiceTests.swift:142: error: -[FirebaseAILogic_Unit_unit.GenerativeAIServiceTests testGenerateContentStream_failure_midStreamError_throwsError] : Asynchronous wait failed: Exceeded timeout of 4 seconds, with unfulfilled expectations: "Stream should throw URLError(.networkConnectionLost)".

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[ncooke3]

The entire contents of the task is in wrapped in a do/catch.

[ncooke3]

This catch block catches the errors from the above task that were previously caught on a per-throwing statement basis.

[ncooke3]

This is new but recommended and required for the new testGenerateContentStream_cancellation_resourceLeak to pass.