fix: add DmScope field to SessionConfig to persist dm_scope setting

The frontend sends dm_scope as part of the session config, but the
backend SessionConfig struct lacked the corresponding field. Go's
encoding/json silently discards unknown fields, so the value was lost
on every PATCH request. Additionally, MarshalJSON only emitted the
session block when Dimensions or IdentityLinks were set, so even a
stored dm_scope would not appear in GET responses.

- Add DmScope string field with json tag 'dm_scope' to SessionConfig
- Update MarshalJSON condition to include session when DmScope is set

pkg/config/config.go

@@ -257,7 +257,7 @@ func (c *Config) MarshalJSON() ([]byte, error) {
 		Alias: (*Alias)(c),
 	}
 
-	if len(c.Session.Dimensions) > 0 || len(c.Session.IdentityLinks) > 0 {
+	if len(c.Session.Dimensions) > 0 || len(c.Session.IdentityLinks) > 0 || c.Session.DmScope != "" {
 		sessionCfg := c.Session
 		aux.Session = &sessionCfg
 	}
@@ -349,6 +349,7 @@ type DispatchSelector struct {
 type SessionConfig struct {
 	Dimensions    []string            `json:"dimensions,omitempty"`
 	IdentityLinks map[string][]string `json:"identity_links,omitempty"`
+	DmScope       string              `json:"dm_scope,omitempty"`
 }
 
 // RoutingConfig controls the intelligent model routing feature.