refactor: modify DELETE API endpoints by returning HTTP 204 No Content + empty body instead of 200 OK + response body with null (#3112)

# What does this PR do?
<!-- Provide a short summary of what this PR does and why. Link to
relevant issues if applicable. -->
The purpose of this PR is to make the behavior DELETE API endpoints be
consistent with standard RESTful conventions and eliminate confusion for
API consumers.

Old Behavior
```
HTTP Status: 200 OK
Response Body: null
```

Eg. `curl -X DELETE http://localhost:8321/v1/shields/test-shield`
`null% `
`INFO 2025-08-12 16:11:57,932 console_span_processor:65 telemetry:
15:11:57.929 [INFO] ::1:59805 - "DELETE /v1/shields/test-shield
HTTP/1.1" 200 `

Updated Behavior
```
HTTP Status: 204 No Content
Response Body: empty (no body)
```

Eg.  `curl -X DELETE http://localhost:8321/v1/shields/test-shield`
`INFO 2025-08-12 16:18:16,645 console_span_processor:62 telemetry:
15:18:16.637 [INFO] ::1:60283 - "DELETE /v1/shields/test-shield
HTTP/1.1" 204 `

<!-- If resolving an issue, uncomment and update the line below -->
<!-- Closes #[issue-number] -->
Closes #3090 

## Test Plan
<!-- Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
re-executed.* -->
Run `./scripts/unit-tests.sh`

llama_stack/core/library_client.py

@@ -380,8 +380,17 @@ class AsyncLlamaStackAsLibraryClient(AsyncLlamaStackClient):
         json_content = json.dumps(convert_pydantic_to_json_value(result))
 
         filtered_body = {k: v for k, v in body.items() if not isinstance(v, LibraryClientUploadFile)}
+
+        status_code = httpx.codes.OK
+
+        if options.method.upper() == "DELETE" and result is None:
+            status_code = httpx.codes.NO_CONTENT
+
+        if status_code == httpx.codes.NO_CONTENT:
+            json_content = ""
+
         mock_response = httpx.Response(
-            status_code=httpx.codes.OK,
+            status_code=status_code,
             content=json_content.encode("utf-8"),
             headers={
                 "Content-Type": "application/json",

llama_stack/core/server/server.py

@@ -21,10 +21,11 @@ from importlib.metadata import version as parse_version
 from pathlib import Path
 from typing import Annotated, Any, get_origin
 
+import httpx
 import rich.pretty
 import yaml
 from aiohttp import hdrs
-from fastapi import Body, FastAPI, HTTPException, Request
+from fastapi import Body, FastAPI, HTTPException, Request, Response
 from fastapi import Path as FastapiPath
 from fastapi.exceptions import RequestValidationError
 from fastapi.responses import JSONResponse, StreamingResponse
@@ -236,6 +237,10 @@ def create_dynamic_typed_route(func: Any, method: str, route: str) -> Callable:
                     result = await maybe_await(value)
                     if isinstance(result, PaginatedResponse) and result.url is None:
                         result.url = route
+
+                    if method.upper() == "DELETE" and result is None:
+                        return Response(status_code=httpx.codes.NO_CONTENT)
+
                     return result
             except Exception as e:
                 if logger.isEnabledFor(logging.DEBUG):