Mock validation (#204)

* Implement mock definition validation and enhance error handling

* Refactor mappingCreateHandler to validate mock definition directly

* Add request path validation to Definition struct

* Fix Validate method receiver type

Change from pointer receiver to value receiver to fix compilation error
in mapping.go where mock.Validate() is called on a value type.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: jmartin82

pkg/mock/definition.go

@@ -97,7 +97,7 @@ type Definition struct {
 	Control     Control  `json:"control"`
 }
 
-func (d *Definition) Validate() error {
+func (d Definition) Validate() error {
 	if d.URI == "" {
 		return fmt.Errorf("URI is required")
 	}