BUG-T8: Fix broker proxy bridge exits after first write due to BaseProtocol missing _drain_helper

[SoundBlaster]

Description

Fixes a silent protocol mismatch in BrokerProxy._make_stdout_writer (src/mcpbridge_wrapper/broker/proxy.py).

asyncio.BaseProtocol was used as the protocol for loop.connect_write_pipe, but asyncio.StreamWriter.drain() calls protocol._drain_helper() which only FlowControlMixin (not BaseProtocol) implements. On every first write the AttributeError was swallowed by _forward_stream's broad except Exception: return, causing the socket→stdout bridge task to exit immediately after the initialize response. asyncio.wait(FIRST_COMPLETED) then cancelled the other direction and the proxy process terminated — so all MCP clients using --broker-spawn or --broker-connect received initialize but never received tools/list, showing 0 tools.

Fix: replace asyncio.BaseProtocol with asyncio.StreamReaderProtocol (inherits FlowControlMixin, implements _drain_helper) — the standard asyncio streams pattern.

Also fixes a pre-existing test isolation bug in TestBrokerProxyBasic: setup_method used BrokerConfig.default() which pointed to ~/.mcpbridge_wrapper/broker.sock. When a live broker was running, the socket existed and test_run_raises_timeout_when_no_socket reached _make_stdin_reader instead of timing out, failing with UnsupportedOperation. Fixed by using a tempfile.mkdtemp() socket path guaranteed not to exist.

Closes / implements: BUG-T8

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactoring
• CI/CD improvement

Quality Gates

• make test - All tests pass with ≥90% coverage (715 passed, 5 skipped; coverage 91.61%)
• make lint - No linting errors (ruff check src/ clean)
• make format - Code is properly formatted
• make typecheck - Type checking passes (mypy src/ — no issues in 18 files)
• make doccheck - Documentation is synced with DocC (if docs changed)

Documentation Sync

• Documentation changes are synced with DocC catalog (or N/A) — no user-facing docs changed

Testing

• Added/updated tests for new functionality — fixed TestBrokerProxyBasic.setup_method test isolation
• All tests pass locally — 715 passed, 5 skipped
• Manually tested the changes — end-to-end proxy session via --broker-connect returns 20 tools through the full initialize → notifications/initialized → tools/list sequence

Checklist

• Code follows the project's style guidelines
• Self-review completed
• Comments added for complex code
• Documentation updated (if needed)
• No new warnings generated
• PR title is descriptive

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