fix: add streaming_run_with_events handler for Gemini Enterprise integration

[anishesg]

Link to Issue or Description of Change

Closes: #770

Problem: Agents deployed to Vertex AI Agent Engine work in the Playground (which calls /api/stream_reasoning_engine with class_method: async_stream_query), but fail when integrated into Gemini Enterprise. Gemini Enterprise sends requests with class_method: streaming_agent_run_with_events, which was not registered in listStreamHandlers in server/agentengine/handler.go. This caused handleQuery in the controller to return "unrecognized class method: streaming_agent_run_with_events".

Solution: Registered a second streamQueryHandler instance for the streaming_agent_run_with_events method name with api_mode: "stream" in listStreamHandlers. The existing streamQueryHandler is fully parameterized by method name and already handles the streaming logic correctly — it just needed to be registered under the additional method name that Gemini Enterprise uses. This mirrors the pattern in the adk-python CLI deploy logic (see cli_deploy.py line 390).

Testing Plan

Unit Tests

• Added or updated unit tests
• All unit tests pass locally

Manual E2E Tests

1. Deploy an adk-go agent to Vertex AI Agent Engine.
2. Integrate the agent into Gemini Enterprise.
3. Submit a query and confirm no "unrecognized class method" error in the logs.
4. Verify the agent responds correctly in Gemini Enterprise.

Local build verification: go build ./server/agentengine/... passes without errors.

Checklist

• Read CONTRIBUTING.md
• Self-review completed
• Code commented (especially complex areas)
• Tests added and passing
• Manual E2E testing completed
• Dependent changes merged/published

Fixes #770

[gemini-code-assist]

Code Review

This pull request adds a new stream query handler, streaming_agent_run_with_events, to the listStreamHandlers function in server/agentengine/handler.go. I have no feedback to provide.

[kdroste-google]

This solution won't work, sorry.
#777 is addressing the same issue - your comments are more than welcome!
I'm closing this PR
Thank you!