fix: add defensive map copies to preserve ObservationContext immutability

Copy the Attributes map in both NewObservationContext (inbound) and the
ObservationContext getter (outbound) to prevent callers from mutating
the internal state of MarketPriceObservation through shared map references.

Author: bjcoombs

services/market-information/domain/observation.go

@@ -227,8 +227,18 @@ func (o MarketPriceObservation) TrustLevel() int {
 }
 
 // ObservationContext returns the typed metadata about collection and processing.
+// Returns a defensive copy to preserve the immutability contract (the Attributes
+// map is a reference type).
 func (o MarketPriceObservation) ObservationContext() ObservationContext {
-	return o.observationContext
+	ctx := o.observationContext
+	if ctx.Attributes != nil {
+		cp := make(map[string]string, len(ctx.Attributes))
+		for k, v := range ctx.Attributes {
+			cp[k] = v
+		}
+		ctx.Attributes = cp
+	}
+	return ctx
 }
 
 // MarketPriceObservationBuilder provides a builder pattern for reconstructing

services/market-information/domain/observation_context.go

@@ -35,11 +35,12 @@ type ObservationContext struct {
 // This is the primary constructor used during observation ingestion where
 // attributes come from the proto AttributeEntry list.
 func NewObservationContext(attributes map[string]string) ObservationContext {
-	if attributes == nil {
-		attributes = make(map[string]string)
+	cp := make(map[string]string, len(attributes))
+	for k, v := range attributes {
+		cp[k] = v
 	}
 	return ObservationContext{
-		Attributes: attributes,
+		Attributes: cp,
 	}
 }