mcp: check that UnmarshalJSON methods for Content don't panic on nil

Introduces a new test file content_nil_test.go which verifies that UnmarshalJSON methods for various Content types do not panic when unmarshaling onto nil pointers.
Adds a nil check in contentFromWire function to guard against a nil wire.Content parameter.
Tests cover different scenarios, including valid and invalid content types, as well as cases with empty or missing content fields.

For #205

mcp/content.go

@@ -252,6 +252,9 @@ func contentsFromWire(wires []*wireContent, allow map[string]bool) ([]Content, e
 }
 
 func contentFromWire(wire *wireContent, allow map[string]bool) (Content, error) {
+	if wire == nil {
+		return nil, fmt.Errorf("content wire is nil")
+	}
 	if allow != nil && !allow[wire.Type] {
 		return nil, fmt.Errorf("invalid content type %q", wire.Type)
 	}