Release: route @custom:<slug> models to their own base_url (#7319) (#7592)

Co-authored-by: n <a@n>

CHANGELOG.md

@@ -9,6 +9,8 @@
 
 ### Fixed
 
+- **Models picked from a second custom provider group route to that provider's own endpoint instead of failing with a 400.** A model selected from a custom provider registered only under `custom_providers:` (picker value like `@custom:omni:antigravity/gemini-3.7-flash-tiered`) was resolved against the active provider's credentials, so it returned `HTTP 400 Invalid model format or no credentials for provider: <bare-model>`. Provider-qualified `@custom:<slug>` ids now resolve to the owning provider's own `base_url` and credentials. A `providers.<slug>` record with no endpoint fails closed with a terminal `CUSTOM_ROUTE_NO_ENDPOINT` (clearing any foreign connection bundle before construction or cache publication) rather than leaking another provider's endpoint, and cross-provider credential/base-URL isolation plus per-profile agent-cache signatures are preserved. Thanks @b3nw. (#7319)
+
 - **Structured (multimodal) message content stored in `state.db` renders as its real content instead of a placeholder, without dropping or misattributing turns.** When the session projection decodes the `state.db` content sentinel, list-valued content now gets an out-of-band identity — a `("structured_content", canonical)` tuple that no scalar string can compare equal to — shared across the merge, dedup, content, and visible reconciliation keys, so an in-band string can't collide with a rich row and an image-only row can no longer vanish from the projection. The expensive canonical serialisation is memoised per content object for the duration of a single `merge_session_messages_append_only()` call (scoped through a `ContextVar`, keyed by object identity with the object held alive so an id can't be reused for a different list mid-call), restoring one serialisation per list per call. Empty/falsy content keys exactly as before. Thanks @totalitarian. (#7492)
 
 - **An empty composer no longer inflates to fit its own placeholder on the JavaScript sizing path.** The fallback resizer measured `scrollHeight` for an empty textarea, which reflects the *placeholder* — so a long busy or compression hint (which wraps to two or three lines) grew the empty composer to roughly 71px, and only after a resize that happened while empty, making the resting height depend on history rather than content. The inline height is now cleared when the value is empty so CSS `min-height` defines the resting size, matching the native path. (#6760)