Add McpTrafficListener for protocol-level observability

[kornys]

Summary

Implement a custom McpTrafficListener (new in 2.0) for MCP protocol traffic observability.

Unlike built-in traffic logging, McpTrafficListener is always invoked regardless of traffic-logging.enabled config. Uses new RawMessage API: id(), method(),
asPrettyString().

Scope

• Create McpTrafficLogger in common/.../observability/ implementing McpTrafficListener
• Log MCP method names, request IDs, and message content at DEBUG level
• isEnabled() returns true — actual logging gated by JBoss Logger level
• Unit test

Acceptance criteria

• McpTrafficLogger created and registered as CDI bean
• DEBUG-level logging of inbound/outbound MCP messages
• Unit test
• CHANGELOG.md updated