feat(deepseek): Add strict-mode support for function calling

[KaparthyReddy]

Description

Implements strict-mode support for DeepSeek's Beta API function calling feature, as requested in #32670.

Changes

• Added strict parameter to ChatDeepSeek class to enable strict mode
• Automatically switches to Beta API endpoint (https://api.deepseek.com/beta) when strict mode is enabled
• Overrode bind_tools() method to add strict: true to tool function definitions when strict mode is active
• Added comprehensive unit tests to verify strict mode functionality

Testing

• All existing tests pass
• Added new test class TestChatDeepSeekStrictMode with tests for:
  • Beta API endpoint switching
  • Default API usage when strict mode is disabled
  • bind_tools() with strict mode enabled
  • Overriding strict mode per tool binding

Usage Example

   from langchain_deepseek import ChatDeepSeek
   from pydantic import BaseModel, Field
   
   class GetWeather(BaseModel):
       location: str = Field(..., description="City and state")
   
   # Enable strict mode
   model = ChatDeepSeek(
       model="deepseek-chat",
       strict=True,  # Uses Beta API with strict mode
   )
   
   # Bind tools with strict schema enforcement
   model_with_tools = model.bind_tools([GetWeather])
   result = model_with_tools.invoke("What's the weather in SF?")

Resolves #32670

[ccurme]

Closing as duplicated with #32727.