Split LangChainChatManager into Inheritance Hierarchy

Description

Refactor the LangChainChatManager class into a base class and model-specific child classes to handle differences between Gemini model versions that leak through the LangChain API.

Context

• Different Gemini model versions have different capabilities and behaviors
• LangChain API doesn't fully abstract these differences
• Model-specific handling is currently mixed in a single class
• Inheritance hierarchy would make model-specific code paths explicit
• Discussion: https://github.com/codeforpdx/tenantfirstaid/pull/222#issuecomment-3848942382

Proposed Structure

class GeminiChatManager:
    """Base class for Gemini-based chat management"""
    # Common functionality across all Gemini models
    
class Gemini2_5ChatManager(GeminiChatManager):
    """Child class for Gemini 2.5-specific features"""
    # Handles Gemini 2.5-specific quirks and features
    
class Gemini3PlusChatManager(GeminiChatManager):
    """Child class for Gemini 3+ specific features"""
    # Handles Gemini 3+ specific quirks and features

Rationale

• Different Gemini model versions have different capabilities and behaviors
• LangChain API doesn't fully abstract these differences
• Model-specific handling is currently mixed in a single class
• Inheritance hierarchy would make model-specific code paths explicit

Benefits

• Clear separation of model-specific logic
• Easier to add support for new Gemini versions
• Better testability of model-specific features
• Reduced conditional logic in core class

Impact

Moderate refactoring of backend/tenantfirstaid/langchain_chat_manager.py and related tests

Priority

🟡 MEDIUM

Parent Issue

#254

References

• PR #222 comment: https://github.com/codeforpdx/tenantfirstaid/pull/222#issuecomment-3848942382
• LangChain Google Generative AI integration docs: https://docs.langchain.com/oss/python/integrations/chat/google_generative_ai
• Current implementation: backend/tenantfirstaid/langchain_chat_manager.py

Start with backend/tenantfirstaid/langchain_chat_manager.py and inspect the related tests to map the current Gemini model-specific behavior and conditional logic. Separate shared behavior from Gemini 2.5 and Gemini 3+ handling according to the proposed hierarchy, then run the related tests to verify the refactor preserves behavior and makes model-specific paths testable.