fix: Dependent bot fixes for python packages

[satyadevai]

Closes #2419

Note

Upgrades FastAPI and Starlette across example backends, relaxes version constraints (incl. pypdf), and updates lockfiles with new transitive deps.

• Dependencies:
  • FastAPI: Bumped to 0.121.0 (locks) and loosened constraints (>=0.118.0) in python/examples/llama-index/backend/pyproject.toml.
  • Starlette: Upgraded to 0.49.3 (locks) and raised constraint to >=0.49.1 in both pyproject.toml files.
  • pypdf: Updated to 6.1.3 and constraint set to >=6.1.3 in python/examples/llama-index/backend/pyproject.toml.
  • Added transitive dependency annotated-doc >=0.0.2 via FastAPI in both lockfiles.
• Lockfiles:
  • Refreshed poetry.lock for both backends with updated hashes and dependency ranges (including httpx extras).

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