fix: Raise StreamableHTTPError on reconnection failure

[wukath]

Motivation and Context

Raise StreamableHTTPError on reconnection failure when the GET stream connection fails and exceeds MAX_RECONNECTION_ATTEMPTS instead of returning silently.

Currently, when the GET stream connection fails and exceeds MAX_RECONNECTION_ATTEMPTS, the transport receive loop returns silently. This prevents the failure from propagating out of the transport receive loop, causing downstream agents/sessions to hang rather than cleanly identifying the disconnection and reporting the failure.

Fixes google/adk-python#2615.

How Has This Been Tested?

• Added test_reconnect_failure_propagates_error to tests/shared/test_streamable_http.py to assert that complete reconnection failure propagates the StreamableHTTPError.
• Executed pytest locally to verify all transport tests pass.

Breaking Changes

No breaking changes. This corrects silent failures and hangs into expected transport exception propagation.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[cubic-dev-ai]

All reported issues were addressed

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[wukath]

hi @maxisbey! was wondering if you could take a look?

[maxisbey]

Thanks for the PR, and sorry it sat here without a proper review.

We're closing most of the open PR backlog. v2 is out and changed a lot of the SDK, so many older PRs no longer apply as written, and we're a small team that realistically doesn't have the capacity to work through the rest.

If this still matters to you on v2, the most useful thing you can do is open an issue (or comment on the existing one) with your use case and a repro. Hearing why it matters to you is what we use to decide what to prioritise.

_{AI Disclaimer}