fix: inject trace context headers per-request instead of at client construction

[ctiliescuuipath]

Summary

• Move W3C trace context header injection from build_uipath_headers() (called once at client init) to per-request hooks in each transport, so headers reflect the active OTEL span at call time
• Legacy OpenAI/Bedrock/Vertex: inject in transport handle_request / handle_async_request
• New clients: inject via _TraceContextHeadersCallback using the dynamic-headers ContextVar
• Add tests for the new callback and _ensure_trace_context_callback helper

Test plan

• Verify test_trace_context_callback.py passes
• Confirm legacy OpenAI, Bedrock, and Vertex transports include trace context headers per-request
• Confirm new client path injects headers via the callback + ContextVar

🤖 Generated with Claude Code

Development Package

• Use uipath pack --nolock to get the latest dev build from this PR (requires version range).
• Add this package as a dependency in your pyproject.toml:

[project]
dependencies = [
  # Exact version:
  "uipath-langchain==0.11.4.dev1008704671",

  # Any version from PR
  "uipath-langchain>=0.11.4.dev1008700000,<0.11.4.dev1008710000"
]

[[tool.uv.index]]
name = "testpypi"
url = "https://test.pypi.org/simple/"
publish-url = "https://test.pypi.org/legacy/"
explicit = true

[tool.uv.sources]
uipath-langchain = { index = "testpypi" }

[tool.uv]
override-dependencies = [
    "uipath-langchain>=0.11.4.dev1008700000,<0.11.4.dev1008710000",
]

[sonarqubecloud]

Quality Gate failed

Failed conditions
70.6% Coverage on New Code (required ≥ 90%)

See analysis details on SonarQube Cloud