Add comprehensive Claude provider improvement plan

Create docs/claude_provider_improvement_plan.md with detailed implementation
plan for AISBF ClaudeProviderHandler improvements identified in the provider
comparison analysis.

Plan includes 10 improvements across 4 phases:
- Phase 1 (Quick Wins): Tool call ID sanitization, empty content filtering,
  prompt caching
- Phase 2 (Core): Thinking block support, tool call streaming, usage metadata
- Phase 3 (Robustness): Message validation, tool result size limits, fallback
- Phase 4 (Advanced): Image/multimodal support

Each improvement includes: problem statement, reference implementation,
detailed implementation steps, files to modify, and effort estimate.

Total estimated effort: 24-37 hours across 4 weeks.