fix(ai): support OpenRouter session affinity (#6496)

Fixes #6366

packages/coding-agent/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Breaking Changes
+
+- Removed the `openai-responses` `compat.sendSessionIdHeader` flag from `models.json`. Session-affinity behavior is now controlled by `compat.sessionAffinityFormat` (`"openai"`, `"openai-nosession"`, or `"openrouter"`). Replace `sendSessionIdHeader: false` with `sessionAffinityFormat: "openai-nosession"` ([#6366](https://github.com/earendil-works/pi/issues/6366)).
+
 ### New Features
 
 - **Cache-friendly dynamic tool loading** - Extensions can add tools during execution while supported Anthropic and OpenAI Responses models preserve prompt-cache prefixes. See [Dynamic Tool Loading](docs/extensions.md#dynamic-tool-loading).
@@ -17,6 +21,7 @@
 ### Fixed
 
 - Fixed inherited OpenRouter model context windows to use the top provider's actual context length ([#6481](https://github.com/earendil-works/pi-mono/pull/6481) by [@davidbrai](https://github.com/davidbrai)).
+- Fixed inherited OpenRouter OpenAI-compatible session IDs to use the `x-session-id` header instead of OpenAI-specific session-affinity fields ([#6366](https://github.com/earendil-works/pi/issues/6366)).
 - Fixed `Ctrl+V` to paste clipboard text when the pasteboard does not contain an image.
 - Fixed `/login amazon-bedrock` to prompt for and save a Bedrock API key instead of only displaying ambient AWS credential setup instructions.