Correct invalid example in management service OpenAPI spec (#2801)

The `example` was incorrectly placed as a sibling of `$ref` within a `schema` object in `polaris-management-service.yml`. According to the OpenAPI specification, properties that are siblings of a `$ref` are ignored.

This was causing a `NullPointerException` in OpenAPI Generator v7.13.0+ due to a change in how examples are processed. The generator now expects all `examples` to be valid and non-empty, and a misplaced `example` can lead to a null reference when the generator tries to access it (we are not yet using v7.13.0+, thus not a problem at the moment).

This commit moves the `example` to be a sibling of the `schema` object, which is the correct placement according to the OpenAPI specification.

Reference error when using newer version of openapi-generator-cli:
```
openapi-generator-cli generate -i spec/polaris-catalog-service.yaml -g python -o client/python --additional-properties=packageName=polaris.catalog --additional-properties=apiNameSuffix="" --skip-validate-spec --additional-properties=pythonVersion=3.13 --ignore-file-override /local/client/python/.openapi-generator-ignore 
...
  Exception: Cannot invoke "io.swagger.v3.oas.models.examples.Example.getValue()" because the return value of "java.util.Map.get(Object)" is null
	at org.openapitools.codegen.DefaultGenerator.processOperation(DefaultGenerator.java:1606)
	at org.openapitools.codegen.DefaultGenerator.processPaths(DefaultGenerator.java:1474)
	at org.openapitools.codegen.DefaultGenerator.generateApis(DefaultGenerator.java:663)
	at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:1296)
	at org.openapitools.codegen.cmd.Generate.execute(Generate.java:535)
	at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)
Caused by: java.lang.NullPointerException: Cannot invoke "io.swagger.v3.oas.models.examples.Example.getValue()" because the return value of "java.util.Map.get(Object)" is null
	at org.openapitools.codegen.utils.ExamplesUtils.unaliasExamples(ExamplesUtils.java:75)
	at org.openapitools.codegen.DefaultCodegen.unaliasExamples(DefaultCodegen.java:2343)
	at org.openapitools.codegen.DefaultCodegen.fromResponse(DefaultCodegen.java:4934)
	at org.openapitools.codegen.DefaultCodegen.fromOperation(DefaultCodegen.java:4575)
	at org.openapitools.codegen.DefaultGenerator.processOperation(DefaultGenerator.java:1574)
	... 6 more
```

Author: MonkeyCanCode

spec/polaris-management-service.yml

@@ -161,12 +161,12 @@ paths:
           application/json:
             schema:
               $ref: "#/components/schemas/CreatePrincipalRequest"
-              example:
-                principal:
-                  name: "alice"
-                  properties:
-                    department: "engineering"
-                credentialRotationRequired: false
+            example:
+              principal:
+                name: "alice"
+                properties:
+                  department: "engineering"
+              credentialRotationRequired: false
       responses:
         201:
           description: "Successful response"