feat: Add robust Hugging Face local model support with GPU memory optimization

## Overview
This PR adds comprehensive support for running Stagehand with local Hugging Face models, enabling on-premises web automation without cloud dependencies. The implementation includes critical fixes for GPU memory management, JSON parsing, and empty result handling.

## Key Features
- **Local LLM Integration**: Full support for Hugging Face transformers with 4-bit quantization (~7GB VRAM)
- **GPU Memory Optimization**: Prevents memory leaks by using shared model instances across multiple operations
- **Robust JSON Extraction**: 5-strategy parsing pipeline with intelligent fallbacks for structured data
- **Content Preservation**: Never loses content - wraps unparseable output in valid JSON structures
- **Graceful Error Handling**: Comprehensive fallback mechanisms prevent empty results

## Technical Improvements

### 1. GPU Memory Management (examples/example_huggingface.py)
- Removed model_name from StagehandConfig to prevent duplicate model loading
- Implemented shared global model instance pattern
- Added cleanup() between examples and full_cleanup() at program end
- Result: Memory stays at ~7GB instead of accumulating to 23GB+

### 2. Enhanced JSON Parsing (stagehand/llm/huggingface_client.py)
- 5-strategy extraction pipeline:
  1. Direct JSON parsing
  2. Pattern matching for extraction fields
  3. Markdown code block extraction
  4. Flexible JSON object detection
  5. Natural language to JSON conversion
- Aggressive prompt engineering for JSON-only output
- Input truncation to prevent CUDA OOM errors
- Fallback responses when model unavailable

### 3. Content Preservation (stagehand/llm/inference.py)
- Critical fix: Wrap raw content in {"extraction": ...} on JSON parse failure
- Prevents content loss during parsing errors
- Ensures no empty results

### 4. Lenient Schema Validation (stagehand/handlers/extract_handler.py)
- Three-tier validation with fallbacks
- Key normalization (camelCase ↔ snake_case)
- Extracts any available string content for DefaultExtractSchema
- Creates valid instances even from malformed data

## Files Modified
- examples/example_huggingface.py: Global model instance pattern
- stagehand/llm/huggingface_client.py: Enhanced JSON parsing and memory management
- stagehand/llm/inference.py: Content preservation on parse failures
- stagehand/handlers/extract_handler.py: Lenient validation with fallbacks
- stagehand/schemas.py: Schema compatibility improvements

## Testing
All 7 examples run successfully:
✅ Basic extraction
✅ Data analysis
✅ Content generation
✅ Multi-step workflow
✅ Dynamic content
✅ Structured extraction
✅ Complex multi-page workflow

## Performance
- Memory: ~7GB VRAM (with 4-bit quantization)
- No CUDA OOM errors
- Zero empty results
- Graceful degradation on errors

## Documentation
Existing HUGGINGFACE_SUPPORT.md provides comprehensive usage guide.

Fixes issues with GPU memory exhaustion, empty extraction results, and JSON parsing failures in local model inference.

Author: kmurad-qlu