fix(elicitation): preserve enumNames through ElicitationSchema serde round-trip (#905)

* fix(elicitation): preserve enumNames through ElicitationSchema serde round-trip

UntitledSingleSelectEnumSchema lacked deny_unknown_fields, so a legacy
enum payload containing enumNames was silently matched by that variant
(ignoring the field) rather than falling through to LegacyEnumSchema.
The enumNames array was lost on re-serialization.

Add deny_unknown_fields to UntitledSingleSelectEnumSchema so that any
unknown field (including enumNames) causes serde to try the next
untagged variant, reaching LegacyEnumSchema correctly.

Also add skip_serializing_if = "Option::is_none" to
LegacyEnumSchema::enum_names so that an untitled legacy enum without
enumNames does not serialize "enumNames": null.

Fixes #903

* test(elicitation): regenerate server schema snapshot for deny_unknown_fields

deny_unknown_fields on UntitledSingleSelectEnumSchema makes schemars
emit additionalProperties: false for that definition. Regenerate the
golden schema fixtures to match (UPDATE_SCHEMA=1 cargo test -p rmcp
--test test_message_schema --all-features).

crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json

@@ -3642,6 +3642,7 @@
           "$ref": "#/definitions/StringTypeConst"
         }
       },
+      "additionalProperties": false,
       "required": [
         "type",
         "enum"

crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema_current.json

@@ -3642,6 +3642,7 @@
           "$ref": "#/definitions/StringTypeConst"
         }
       },
+      "additionalProperties": false,
       "required": [
         "type",
         "enum"