fix: Replace remaining manual server polling with wait_for_server helper

[maxisbey]

This PR completes the work started in #1527 by fixing the remaining test fixtures that were still using manual socket polling loops to wait for server startup.

Motivation and Context

PR #1527 introduced the wait_for_server() helper and fixed most test files, but missed 4 test fixtures that were still using manual polling loops with arbitrary time.sleep() calls. These remaining fixtures could still cause flaky test failures in CI due to race conditions.

Changes Made

Fixed 4 test fixtures that still used manual polling:

1. tests/server/fastmcp/test_integration.py - server_transport fixture
2. tests/client/test_notification_response.py - non_sdk_server fixture
3. tests/client/test_http_unicode.py - running_unicode_server fixture
4. tests/shared/test_sse.py - mounted_server and context_server fixtures

Before (Manual Polling):

max_attempts = 20
attempt = 0
while attempt < max_attempts:
    try:
        with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
            s.connect(("127.0.0.1", server_port))
            break
    except ConnectionRefusedError:
        time.sleep(0.1)  # Arbitrary delay
        attempt += 1
else:
    raise RuntimeError(f"Server failed to start after {max_attempts} attempts")

After (Active Polling):

wait_for_server(server_port)  # Returns immediately when ready

How Has This Been Tested?

All modified test files have been verified:

• pytest tests/server/fastmcp/test_integration.py - 20 tests pass
• pytest tests/client/test_notification_response.py - 1 test passes
• pytest tests/client/test_http_unicode.py - 2 tests pass
• pytest tests/shared/test_sse.py::test_sse_client_basic_connection_mounted_app - 1 test passes

Breaking Changes

None - this is purely an internal test infrastructure improvement.

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

This change:

• Removes 49 lines of duplicate manual polling code
• Makes tests more deterministic and reliable
• Reduces wasted time from fixed delays (tests return as soon as server is ready)

All server startup waiting uses the centralized wait_for_server() helper.

[felixweinberger]

🧑‍🍳 👌