Port PR #685 fixes for HTTP 400 on Together AI and strict OpenAI-compat hosts,
with additional improvements:
- Gate reasoning_content on assistant history to allowlisted models only
(OpenAI o-series/GPT-5, DeepSeek, Kimi) — prevents HTTP 400 on Together/Qwen
- Gate reasoning_effort to models that support it (OpenAI reasoning family)
- Skip stream_options for Together endpoints (causes HTTP 400)
- Scope DashScope enable_thinking/thinking_budget to DashScope providers only
- Reorder multimodal parts: text before images (Together/Qwen preferred order)
- Add redacted_thinking tag to sanitization patterns with early-exit guard fix
- Upgrade Together detection from URL-only to URL + providerType + name fallback
(mirrors dashScopePassthroughKeys pattern for reverse-proxy compatibility)
- Add comprehensive tests for all new behavior
