Add tests for resource leak in streamable_http SSE handlers

[certainly-param]

Fixes #1450

I found that HTTP responses don't get closed properly when SSE streaming fails with exceptions in the streamable HTTP client.

The Problem:
When the async for loop in _handle_sse_response and _handle_resumption_request throws an exception (malformed JSON, network error, etc.), the response is never closed because response.aclose() is only called in the success path.

Impact:
This can cause connection pool exhaustion in long-running clients, eventually causing new requests to hang or fail.

What I Added:

• test_resource_leak_reproduction.py - Standalone script that demonstrates the issue
• tests/client/test_streamable_http_resource_leak.py - Pytest tests that verify the resource leak and show how the fix should work

Both test files show that when SSE parsing fails, the response doesn't get closed, confirming the resource leak.

The Fix:
The methods need finally blocks to ensure response.aclose() is always called:

try:
    # ... existing code ...
except Exception as e:
    # ... existing exception handling ...
finally:
    await response.aclose()

This ensures resources are properly cleaned up regardless of whether exceptions occur.