[Feature]: Implement Provider Strategy Pattern for Multi-Model LLM Support

### Is your feature request related to a problem? Please describe.

Provider strategy pattern created for different LLM model families (GPT-5, GPT-4o, LM Studio, Ollama) with automatic routing based on model prefixes.
Scenario generation failing due to JSON parsing errors when using GPT-5 models. Character background isn't created correctly on gpt-5 models.

The scenario generation system fails when using GPT-5 models (gpt-5-nano, gpt-5-mini) due to malformed JSON responses that cannot be parsed by the fallback JSON extraction logic.
Error Details:
fallback
Root Cause: GPT-5 models generate JSON responses with formatting issues (unterminated strings, extra commas, malformed structures) that the current simple extraction logic cannot handle.

### Describe the solution you'd like

1. Provider Capabilities Matrix (backend/providers/capabilities.py)
Model family detection and capability mapping
API family routing (responses vs chat_completions vs openai_compatible)
2. Strategy Pattern Implementation
OpenAIGPT5Strategy: GPT-5 Responses API integration
OpenAIGPT4oStrategy: GPT-4o Chat Completions via LangChain
OpenAICompatibleStrategy: LM Studio/Ollama support
3. Provider Routing (backend/providers/openai.py)
Automatic strategy selection based on model name prefix
Unified ProviderResponse interface

Testing - Test suite needs to be updated for new architecture

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

### Would you like to contribute this feature?

- [ ] Yes, I would like to implement this feature

The issue describes a strategy pattern for LLM providers. Start by examining backend/providers/capabilities.py and backend/providers/openai.py to understand the current structure. The goal is to create new strategy classes (OpenAIGPT5Strategy, OpenAIGPT4oStrategy, OpenAICompatibleStrategy) that handle different API families and JSON response formats. Testing will need updates to cover the new architecture.

索引モデルが issue の本文から書いたものです。