feat: add event validation

Validates required envelope fields: schema, event.id, event.type,
event.ts, and event.source (framework, client_id, host).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/event/validate.go

@@ -0,0 +1,85 @@
+package event
+
+import (
+	"errors"
+	"fmt"
+)
+
+var validTypes = map[string]bool{
+	"session.start":   true,
+	"session.end":     true,
+	"run.start":       true,
+	"run.end":         true,
+	"span.start":      true,
+	"span.end":        true,
+	"error":           true,
+	"metric.snapshot": true,
+}
+
+type ValidationError struct {
+	Field   string
+	Message string
+}
+
+func (e ValidationError) Error() string {
+	return fmt.Sprintf("%s: %s", e.Field, e.Message)
+}
+
+func Validate(m map[string]any) error {
+	// Check schema
+	schema, ok := m["schema"].(map[string]any)
+	if !ok {
+		return ValidationError{Field: "schema", Message: "missing or invalid"}
+	}
+	if name, _ := schema["name"].(string); name != "agentmon.event" {
+		return ValidationError{Field: "schema.name", Message: "must be 'agentmon.event'"}
+	}
+	if ver, _ := schema["version"].(float64); ver != 1 {
+		return ValidationError{Field: "schema.version", Message: "must be 1"}
+	}
+
+	// Check event
+	event, ok := m["event"].(map[string]any)
+	if !ok {
+		return ValidationError{Field: "event", Message: "missing or invalid"}
+	}
+
+	if id, _ := event["id"].(string); id == "" {
+		return ValidationError{Field: "event.id", Message: "required"}
+	}
+
+	eventType, _ := event["type"].(string)
+	if eventType == "" {
+		return ValidationError{Field: "event.type", Message: "required"}
+	}
+	if !validTypes[eventType] {
+		return ValidationError{Field: "event.type", Message: fmt.Sprintf("invalid type '%s'", eventType)}
+	}
+
+	if event["ts"] == nil {
+		return ValidationError{Field: "event.ts", Message: "required"}
+	}
+
+	// Check source
+	source, ok := event["source"].(map[string]any)
+	if !ok {
+		return ValidationError{Field: "event.source", Message: "missing or invalid"}
+	}
+
+	if fw, _ := source["framework"].(string); fw == "" {
+		return ValidationError{Field: "event.source.framework", Message: "required"}
+	}
+	if cid, _ := source["client_id"].(string); cid == "" {
+		return ValidationError{Field: "event.source.client_id", Message: "required"}
+	}
+	if host, _ := source["host"].(string); host == "" {
+		return ValidationError{Field: "event.source.host", Message: "required"}
+	}
+
+	return nil
+}
+
+func IsValidationError(err error) bool {
+	var ve ValidationError
+	return errors.As(err, &ve)
+}