AI: Failed to parse stream (AI/parse-failed)

### Operating System

macOS

### Environment (if applicable)

Chrome

### Firebase SDK Version

12

### Firebase SDK Product(s)

AI

### Project Tooling

Expo with Firebase web js.

### Detailed Problem Description

I am giving the model a prompt which is expected to generate a very long response. I have tried non-streaming and streaming mode and the failure occurs exactly after 3 minutes. Firebase transactions have 3 minute timeout but I am not using them. I have set the timeout to 20 minutes for the model.

In non-streaming mode the request failed with 502 error and in streaming mode I stop getting new Eventstream messages and a `FirebaseError: AI: Failed to parse stream (AI/parse-failed)`.

### Steps and code to reproduce issue

Prompt requesting long response.

Gemini 2.5 pro, thinking budget 32728, safety: block none, timeout: 1200 * 1e3

The report names no source files or tests; begin by locating the Firebase AI streaming implementation and its handling of EventStream messages and parse failures. Reproduce with Gemini 2.5 Pro, the stated thinking budget, and a prompt producing a very long response, then verify that streaming does not fail after three minutes and that the non-streaming request no longer returns 502.

Written by the indexing model from the issue text.