Implement support for OpenRouter sticky routing via a session_id variable

internal/domain/domain.go

@@ -47,6 +47,11 @@ const (
 	CacheControlEphemeral CacheControlType = "ephemeral"
 )
 
+const (
+	// SessionIDMaxLength is OpenRouter's documented maximum session_id length.
+	SessionIDMaxLength = 256
+)
+
 // CacheControl describes provider cache metadata attached to prompt content.
 type CacheControl struct {
 	Type CacheControlType `yaml:"type" json:"type"`
@@ -98,6 +103,7 @@ type PreparedRun struct {
 	OutputContract       OutputContract        `json:"output_contract"`
 	StructuredOutput     *StructuredOutputSpec `json:"structured_output,omitempty"`
 	InputHashes          map[string]string     `json:"input_hashes,omitempty"`
+	SessionID            string                `json:"session_id,omitempty"`
 	RenderedPromptHash   string                `json:"rendered_prompt_hash"`
 	Messages             []RenderedMessage     `json:"messages"`
 	StartTime            time.Time             `json:"start_time,omitempty"`
@@ -128,6 +134,7 @@ type PromptDefinition struct {
 	Version        string                  `yaml:"version"`
 	DefaultProfile string                  `yaml:"default_profile"`
 	Description    string                  `yaml:"description"`
+	SessionID      string                  `yaml:"session_id" json:"session_id,omitempty"`
 	Inputs         []PromptInput           `yaml:"inputs"`
 	Templates      []PromptMessageTemplate `yaml:"templates"`
 	OutputFormat   OutputFormat            `yaml:"output_format"`
@@ -189,7 +196,8 @@ type OutputContract struct {
 
 // RenderedPrompt represents the prompt after template application.
 type RenderedPrompt struct {
-	Messages []RenderedMessage `json:"messages"`
+	SessionID string            `json:"session_id,omitempty"`
+	Messages  []RenderedMessage `json:"messages"`
 }
 
 // RenderedMessage is a single message in a rendered prompt.