feat: add comprehensive configuration system and advanced tagging tests

This commit introduces a complete YAML-based configuration system, prompt
templates, tag hierarchies, and comprehensive tests for advanced features.

## Configuration System (5 YAML files)

- config/model_config.yaml (314 lines added):
  * Complete LLM provider configurations (Ollama, Gemini, Cerebras)
  * Model-specific parameters and defaults
  * Routing rules and fallback chains
  * Performance tuning settings
  * Cost and latency parameters

- config/enhanced_prompts.yaml:
  * Enhanced prompt templates for quality improvement
  * Multi-stage extraction prompts
  * Context-aware prompt variations
  * Specialized prompts for different document types

- config/custom_tags.yaml:
  * Custom tag definitions
  * Tag metadata and descriptions
  * Tag grouping and categories
  * Validation rules

- config/document_tags.yaml:
  * Document classification tags
  * Domain-specific tag sets
  * Tag aliases and synonyms
  * Tag usage guidelines

- config/tag_hierarchy.yaml:
  * Hierarchical tag structure
  * Parent-child relationships
  * Tag inheritance rules
  * Category organization

## Prompt Templates (2 YAML files)

- data/prompts/few_shot_examples.yaml:
  * Curated few-shot learning examples
  * Category-specific examples
  * High-quality example selection
  * Performance-validated examples

- data/prompts/few_shot_examples.yaml.bak:
  * Backup of prompt examples
  * Version history preservation

## Advanced Tests (4 test files)

- test/integration/test_advanced_tagging.py:
  * Tag hierarchy testing
  * Multi-label tagging validation
  * Custom tag integration
  * Monitoring and metrics
  * A/B testing integration
  * End-to-end tagging workflow

- test/unit/test_ai_processing_simple.py:
  * AI component error handling
  * Vision processor tests
  * AI enhancement validation

- test/unit/test_config_loader.py:
  * Configuration loading tests
  * YAML parsing validation
  * Default value handling
  * Environment variable integration

- test/unit/test_ollama_client.py:
  * Ollama client functionality
  * Local LLM integration
  * Model loading and inference
  * Error handling and retries

## Key Features

1. **Flexible Configuration**: YAML-based config for easy customization
2. **Multi-Provider Support**: Unified config for all LLM providers
3. **Tag System**: Hierarchical, multi-label document tagging
4. **Prompt Library**: Reusable, tested prompt templates
5. **Comprehensive Testing**: Integration and unit tests for all features

## Configuration Highlights

Model configs for:
- Ollama: llama3.2, mistral, qwen2.5
- Gemini: gemini-1.5-flash, gemini-1.5-pro
- Cerebras: llama3.1-8b, llama3.3-70b

Features:
- Automatic routing based on task type
- Cost optimization settings
- Performance tuning parameters
- Fallback chains for reliability

## Tag System Benefits

- Automatic document classification
- Multi-label support (one doc, many tags)
- Hierarchical organization
- ML-based prediction
- Custom tag extensions

Implements Phase 2 configuration and advanced tagging capabilities.

Author: vinod0m