Move profile composition to the engine facade

types.go

@@ -440,11 +440,12 @@ type ExecutionTargetOverride struct {
 // use profile YAML api_key_env with file and FS profile sources. Profile has no
 // stable JSON representation.
 //
-// WithProfiles validates and copies Profile values during NewEngine. Optional
-// numeric zero, blank strings, and an empty ExtraParams map inherit
-// lower-precedence values. An optional provider control that remains zero is
-// unspecified; use ExecutionTargetOverride pointer fields to request an
-// explicit numeric zero.
+// WithProfiles validates and copies Profile values during NewEngine. Zero
+// Temperature, MaxTokens, and TopP values and blank ServiceTier and
+// ReasoningEffort values leave those provider controls unspecified. A zero
+// TimeoutSeconds retains the framework deadline, while an empty ExtraParams map
+// inherits backend request defaults. Use ExecutionTargetOverride pointer fields
+// to request an explicit numeric zero.
 type Profile struct {
 	// ID is the required non-blank profile identifier. WithProfiles trims it.
 	ID string
@@ -458,23 +459,19 @@ type Profile struct {
 	Endpoint string
 	// Model is the required non-blank provider model identifier.
 	Model string
-	// Temperature is from 0 through 2. Zero inherits a lower-precedence value
-	// and otherwise leaves the provider control unspecified.
+	// Temperature is from 0 through 2. Zero leaves the provider control
+	// unspecified.
 	Temperature float64
-	// MaxTokens is non-negative. Zero inherits a lower-precedence value and
-	// otherwise leaves the provider control unspecified.
+	// MaxTokens is non-negative. Zero leaves the provider control unspecified.
 	MaxTokens int
-	// TopP is from 0 through 1. Zero inherits a lower-precedence value and
-	// otherwise leaves the provider control unspecified rather than selecting an
-	// explicit zero.
+	// TopP is from 0 through 1. Zero leaves the provider control unspecified
+	// rather than selecting an explicit zero.
 	TopP float64
-	// TimeoutSeconds is non-negative. Zero inherits a lower-precedence value and
-	// otherwise the framework deadline.
+	// TimeoutSeconds is non-negative. Zero retains the framework deadline.
 	TimeoutSeconds int
-	// ServiceTier is optional; a blank value inherits a lower-precedence value.
+	// ServiceTier is optional; a blank value leaves it unspecified.
 	ServiceTier string
-	// ReasoningEffort is optional; a blank value inherits a lower-precedence
-	// value.
+	// ReasoningEffort is optional; a blank value leaves it unspecified.
 	ReasoningEffort string
 	// APIKeyRequired clears a backend's inherited API-key environment name and
 	// requires a non-blank RunRequest.APIKey unless the request explicitly