fix: prevent infinite reconnection loop when SSE stream drops without response

[dragogargo]

Motivation and Context

Fixes #2393. _handle_reconnection() resets the attempt counter to 0 when a reconnection succeeds at the HTTP level but the stream ends without delivering a complete JSON-RPC response. This makes MAX_RECONNECTION_ATTEMPTS ineffective — the counter only applies to consecutive exceptions, not total reconnection attempts. If the server accepts the connection but the stream drops repeatedly (sending only priming events), the client retries forever.

In production this causes MCP client coroutines to hang indefinitely when a server experiences transient stream drops.

How Has This Been Tested?

• Regression test (test_handle_reconnection_stops_after_max_attempts) that mocks aconnect_sse to simulate a server sending only priming events then dropping. Verifies the client gives up after MAX_RECONNECTION_ATTEMPTS and sends a JSONRPCError back to the caller.
• All 62 existing test_streamable_http.py tests pass, including test_streamable_http_multiple_reconnections (3 legitimate reconnections with real data).

Changes

1. Smart attempt counter: track whether real SSE data (not just priming events) was received during reconnection. Reset counter only when progress was made; increment when server only sends empty priming events.
2. Error reporting: when max attempts are exceeded, send a JSONRPCError back through read_stream_writer so call_tool returns an error instead of hanging forever.
3. Regression test: in-memory unit test with mocked SSE to verify the fix.

Breaking Changes

No.

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