fix(deepseek): handle array inputs

[davecrunch]

Description

Fixes issue where Deepseek's ToolMessage field does not accept array input. Created workaround by converting array inputs into strings. Adjusted relevant test cases.

Files modified:

libs/partners/deepseek/langchain_deepseek/chatmodels.py
libs/partners/deepseek/tests/unit_tests/test_chat_models.py

Fixes #33965

Used Cursor for assistance

[davecrunch]

Hi, I'm running into this issue in the CI where the deepseek api key is not set. I'm not sure where this issue came up, I don't think it's introduced through my commits. Would appreciate some guidance on this, thank you! @mdrxy @ccurme