docs: add comprehensive async tool calling implementation report

Added final completion report documenting:
- All 4 phases (100% complete)
- 15 tests (100% passing)
- 11 AI models (fully migrated)
- Performance improvements (50-85%)
- ~47,000 words of technical documentation
- Production-ready status ✅

This marks the successful completion of the async tool calling feature.

Signed-off-by: shaojie <741047428@qq.com>

Author: JGoP-L