refactor: move FastAPI active thread tracking into Starlette

[faizansaiyed123]

Ref #7753

This separates the active-thread/profiler bookkeeping from the FastAPI-specific request handling.

Changes:

• Centralize active-thread updates in the Starlette integration.
• Provide a Starlette sync-handler wrapper so active-thread metadata is updated in the thread that actually executes a synchronous handler.
• Reuse that wrapper from the FastAPI integration.
• Preserve both static transaction and span-streaming behavior.

The follow-up FastAPI PR uses this Starlette helper while replacing the private FastAPI request-handler patch with a global dependency.

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