Fix get_collection/create_collection Raises docstring (#5492)

[adityasingh2400]

Description of changes

The Raises section on ClientAPI.get_collection says ValueError, but the implementation in chromadb/api/segment.py actually raises NotFoundError. The same mismatch shows up on create_collection, whose Raises lists ValueError for duplicate names while the SysDB / gRPC path raises UniqueConstraintError. The async client and the concrete Client wrapper have the same drift, so callers consulting any of the three files get the wrong exception type to catch.

This PR updates the docstrings in chromadb/api/__init__.py, chromadb/api/async_api.py, and chromadb/api/client.py so the sync and async clients describe the exceptions that are actually raised:

• get_collection: ValueError becomes NotFoundError.
• create_collection: the duplicate-name entry becomes UniqueConstraintError; the existing ValueError: If the collection name is invalid. entry is kept since name validation does raise ValueError.
• Client.get_collection and Client.create_collection: add the missing NotFoundError / UniqueConstraintError entries alongside the existing embedding-function ValueError entry, which is still raised by validate_embedding_function_conflict_on_create / _on_get.

Both NotFoundError and UniqueConstraintError are already exported from chromadb/errors.py, so users can from chromadb.errors import NotFoundError, UniqueConstraintError and catch them today.

Fixes #5492.

This also supersedes the Python portion of #6369, which has been idle for roughly three months and bundles unrelated .github/workflows/_rust-tests.yml GCS credential changes that aren't appropriate for a docs fix.

Test plan

Docstring-only change with no runtime behavior. Verified by reading chromadb/api/segment.py (raise NotFoundError(...) at line 338 for get_collection) and chromadb/db/impl/grpc/client.py (raise UniqueConstraintError() for duplicate-collection paths), and confirming the existing ValueError raises in chromadb/api/client.py come from validate_embedding_function_conflict_on_create / _on_get in chromadb/api/collection_configuration.py.

Documentation Changes

This PR is itself the documentation change. No updates needed in the docs repo.

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