Observability Phase 2 - Tracing & Errors: Journey 1 complete

[david-martin]

Building on #429, implement tracing and errors as per the observability plan in #350

• Add span attributes:
  • mcp.tool, mcp.server, http.status_code, component, decision.allowed.
• Capture immediate_response (ext-proc 404) as span events.
• Add error source markers:
  • Envoy: response_code_details
  • ext-proc: error_source="ext-proc"
  • broker/server: standard OTel HTTP spans + error_source="backend"

Outcome: A user can click one trace and visibly see where a 404 appeared and why.

For reference, examples of adding spans in the kuadrant operator https://github.com/Kuadrant/kuadrant-operator/pull/1704/files