feat: Enable configurable streaming support for OpenAI models

[syedazeez337]

Description:

This PR addresses the issue where requests to OpenAI-compatible model
services were hardcoded with "stream": false.

I have introduced a new stream configuration field to the Agent API. This gives
developers explicit control over whether they want streaming responses enabled or
disabled for their agents.

Key Changes:

   - API Update: Added an optional stream field (boolean) to the Agent spec in both
     v1alpha1 and v1alpha2.
   - Default Behavior: To ensure backward compatibility and safety, the field defaults
     to false if not specified.
   - ADK Integration: Updated the Python ADK and the translator logic to propagate
     this setting all the way down to the model runner. When enabled, it correctly
     sets the internal state to StreamingMode.SSE.
   - Documentation: Updated the Helm chart descriptions to reflect the new field.

Testing:

   - Added new Python unit tests to verify that the boolean flag correctly maps to the
     StreamingMode enum.
   - Verified that existing OpenAI model integration tests pass.
   - Updated the Go "golden files" to match the new API schema.

Everything has been linted and verified against the contribution guidelines. Let me
know if you have any questions

Fixes #1099

[EItanya]

Thanks so much for the contribution, and this change definitely makes sense. The one thing i'd like to note is that I actually originally tried to make stream: true the default, but ran into issues. Have you done e2e testing with stream = true?

[syedazeez337]

Hi @EItanya, I have tested and passed the e2e tests locally. Let me know if it is working now. Thank you.