Harden backend contracts and documentation

docs/integrations/openai-compatible-chat.md

@@ -13,10 +13,15 @@ that produce these outbound settings.
 ## Endpoint And Method
 
 Generation sends an HTTP `POST` with `Content-Type: application/json`.
-A non-empty endpoint from the execution target overrides the client's
-configured base URL. After trailing slashes are removed,
-`/chat/completions` is appended. Generation fails before sending when neither
-source supplies an endpoint.
+Before the client is called, the engine resolves framework, backend, profile,
+and request values into one execution target. A non-empty endpoint from that
+target overrides the client's configured base URL. After trailing slashes are
+removed, `/chat/completions` is appended. Generation fails before sending when
+neither source supplies an endpoint.
+
+The target's backend ID is routing metadata for prepared values, results, and
+injected clients. The built-in client does not derive the URL from that ID and
+does not serialize it in the provider request.
 
 ## Authentication
 
@@ -26,6 +31,12 @@ the client reads that variable and requires a non-empty value. The selected
 key is sent as `Authorization: Bearer <key>`. No authorization header is sent
 when neither mechanism is configured.
 
+The target contains the already resolved environment-variable name: an
+explicit request override takes precedence over profile metadata, which takes
+precedence over the backend default. Only the name reaches prepared metadata;
+the environment value is read just before the provider call and is never added
+to the JSON body.
+
 ## Request Body
 
 The request body always contains `model` and `messages`. The execution
@@ -47,9 +58,11 @@ The client conditionally includes:
 - `response_format` for JSON Schema structured output, including its name,
   strict flag, and schema document.
 
-Extra parameters are merged directly into the top-level body after JSON
-serialization is verified. Empty keys and collisions with these reserved
-fields are rejected before any provider call:
+The engine resolves backend, profile, and request extra-parameter maps by
+whole-map replacement rather than key merging. The resulting effective map is
+then merged directly into the top-level body after JSON serialization is
+verified. Empty keys and collisions with these reserved fields are rejected
+before any provider call:
 
 - `model`
 - `session_id`
@@ -61,6 +74,9 @@ fields are rejected before any provider call:
 - `reasoning_effort`
 - `response_format`
 
+`backend_id`, `api_key_env`, and resolved credential values are not provider
+request fields.
+
 ## Response Handling
 
 Any 2xx response is decoded as an OpenAI-compatible chat response. The client