fix: lazy bedrock import and BYO Anthropic token counting

[andreiancuta-uipath]

What changed and why

Fixes two regressions introduced in 1.15.1 (#103):

1. Package unimportable without the bedrock extra. factory.py imported clients.bedrock.model_resolution at module level; importing that submodule executes clients/bedrock/__init__.py, which imports the extras-guarded chat_models.py — so import uipath_langchain_client raised ImportError: The 'bedrock' extra is required... for any install without langchain-aws/anthropic. CI didn't catch it because it installs --all-extras.

   • Fix: Extract the model_resolution module outside of the bedrock/ module, since it's actually pure and does not depend on the bedrock extra dependency.

2. Added a test and CI step that tries to import the package without the extras installed. This should catch future regressions of this sort.

Affected packages

uipath-langchain-client 1.15.1

Verification

• Both new tests watched failing with the exact regression errors before the fixes, passing after.
• ruff check, ruff format, pyright: clean.
• Full suite failure set is byte-identical to origin/main (local environment-only failures; zero introduced).

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