fix: preserve Xiaomi MiMo thinking blocks

[hirogen317]

Summary

• Add a Xiaomi MiMo Anthropic-compatible endpoint detector for *.xiaomimimo.com/anthropic.
• Preserve unsigned reasoning_content-derived thinking blocks for Xiaomi MiMo replay turns, matching the existing Kimi/DeepSeek strip-signed/keep-unsigned policy.
• Add a specification-driven development note plus focused regression coverage for Xiaomi MiMo, signed-block stripping, cache-control stripping, and non-Xiaomi third-party behavior.

Test Plan

• RED observed before implementation: python -m pytest --tb=short -q tests/agent/test_xiaomi_mimo_anthropic_thinking.py failed with missing detector / thinking blocks stripped.
• /Users/gen/.hermes/hermes-agent/venv/bin/python -m pytest --tb=short -q tests/agent/test_xiaomi_mimo_anthropic_thinking.py tests/agent/test_deepseek_anthropic_thinking.py → 18 passed.
• /Users/gen/.hermes/hermes-agent/venv/bin/python -m pytest --tb=short -q tests/agent/test_xiaomi_mimo_anthropic_thinking.py tests/agent/test_deepseek_anthropic_thinking.py tests/agent/test_kimi_coding_anthropic_thinking.py → 35 passed.
• /Users/gen/.hermes/hermes-agent/venv/bin/python -m ruff check agent/anthropic_adapter.py tests/agent/test_xiaomi_mimo_anthropic_thinking.py → All checks passed.

Docker / containerization

• No Dockerfile, dependency, or environment-variable changes were needed; the existing container image copies these Python source/test changes normally.
• Local docker build -t hermes-agent:issue-24884 . was attempted but failed before project build steps while resolving debian:13.4 from Docker Hub due local Docker proxy DNS (lookup http.docker.internal ... connection refused). This appears environment/network-related rather than code-related.

Closes #24884

[alt-glitch]

Duplicate of #24465 — 9th+ competing PR for Xiaomi MiMo reasoning_content echo-back. #24465 is the earliest open PR with 26 tests and comprehensive coverage.