feat: Add Langfuse observability integration with optional decorators…

[achuajays]

…, client, and updated documentation.

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Summary by cubic

Add optional Langfuse observability with decorators and a client to trace functions and async generators. Includes helper utilities, exports, and docs; acts as a no-op when Langfuse isn’t installed.

• New Features

  • observe_langfuse and observe_langfuse_debug decorators (span/generation), with input/output capture and async generator support; debug mode via LANGFUSE_DEBUG/BU_DEBUG.
  • LangfuseClient wrapper and helpers: is_langfuse_available, flush_langfuse.
  • init exports and README examples; Laminar observability remains available as an alternative.

• Dependencies

  • Added optional extra: langfuse>=2.0.0 (install with uv add bu-agent-sdk[langfuse]); expects LANGFUSE_SECRET_KEY and LANGFUSE_PUBLIC_KEY.

^{Written for commit f61bd57. Summary will update on new commits.}

[cubic-dev-ai]

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