fix(platform): use LLMOps span ID in trace header

[cfauchere]

Summary

• _inject_trace_context now prefers the external (LLMOps) span over the raw OTEL span when building the x-uipath-traceparent-id header
• This ensures the span ID in the header matches what's visible in the LLMOps trace UI, enabling end-to-end correlation between agent traces and downstream services (ECS / Context Grounding)
• Previously the header carried @traced SDK span IDs which are not in the LLMOps trace, making correlation impossible

Test plan

• Local validation: ran agent with CG tool, verified header span ID 95eb5d6c5cec2544 matches the test (contextGroundingTool) span in the LLMOps trace
• Trace ID matches across all headers
• No Failed to detach context errors
• Agent completes successfully with CG search results

Validation

  contextgrounding_unified_search  span=22196e2e852ae856  parent=72345458d68098f9
  ├── contextgrounding_retrieve    span=c59f6793ad7ec559  parent=22196e2e852ae856
  │   ├── folder_retrieve_key      span=1257f581d112efd0  parent=c59f6793ad7ec559
  │   │   └── GET (folders)        span=50b5af2c58e2bad1  parent=1257f581d112efd0
  │   └── GET (indexes)            span=0db3fdbe58f0b31c  parent=c59f6793ad7ec559
  └── POST (search)                span=71d8fe76755ccea9  parent=22196e2e852ae856 

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