P2-T4: Surface broker unavailability as JSON-RPC error instead of silent timeout

[SoundBlaster]

Description

When BrokerProxy cannot connect to the broker (timeout, spawn failure, connection refused), the client previously received no response and hung silently — showing "0 tools" or a generic connection error with no actionable message.

Fix: wrap the connect phase in run() with a try/except that catches connection failures and writes a JSON-RPC 2.0 error response (code: -32001, message: "Broker unavailable: <reason>") to stdout before returning. The client receives a meaningful error immediately instead of hanging until its own timeout fires.

Changes:

• src/mcpbridge_wrapper/broker/proxy.py — added import json; added _send_broker_error() helper; wrapped connect phase in run() with try/except
• tests/unit/test_broker_proxy.py — added TestBrokerProxyUnavailableError (5 tests); updated existing timeout test to verify new behaviour
• tests/unit/test_broker_stubs.py — updated test_run_raises_timeout_when_no_socket → test_run_writes_error_when_no_socket to match new behaviour

Type of Change

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

Quality Gates

• make test - All tests pass with ≥90% coverage (732 passed, 91.59% coverage)
• make lint - No linting errors (ruff check src/ passed)
• make format - Code is properly formatted (ruff format --check passed)
• make typecheck - Type checking passes
• make doccheck - Documentation is synced with DocC (if docs changed)

Documentation Sync

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

Testing

• Added/updated tests for new functionality (5 new tests in TestBrokerProxyUnavailableError, 2 existing tests updated)
• All tests pass locally (732 passed, 0 failures)
• Manually tested the changes

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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