feat: harden cache reuse semantics and expand protocol regressions

Stabilize cross-protocol ask-mode/streaming behavior and reduce session-reuse branch collisions, then add focused docs/tests for multimodal normalization and pool/stats/config paths.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

README.md

@@ -5,6 +5,7 @@
 - OpenAI：`/v1/models`、`/v1/chat/completions`（含 stream）
 - Anthropic：`/v1/messages`、`/v1/messages/count_tokens`（含 stream）
 - 内置：多实例池、会话复用、Prometheus 指标、登录态 bundle 注入
+- 多模态降级：OpenAI `image_url` / `input_image` 转 `[image]`，`input_audio` 转 `[audio]`；Anthropic `image` 转 `[image]`
 
 > 架构设计与二开细节请看 [`DESIGN.md`](./DESIGN.md)。