fix(apiserver): abort the request on every middleware failure (bug-1949)

Cause: ReturnError only writes the response and never aborts the gin
chain; RequirePermission (token error) and WhitelistIPMiddleware (invalid
client IP) returned after ReturnError without ctx.Abort(), so the route
handler still ran after the rejection.

Fix: ctx.Abort() after ReturnError at both sites (and, for clarity, on the
ValidateHandler body-check path, which validateRequestBody already
aborted). RequirePermission now answers a bad token with 401 instead of
500. ReturnError stays non-aborting (documented): handlers such as
ControlHost call it from helpers and then test ctx.IsAborted(), so a
global abort would change handler flows, and it adds nothing in handlers.

Tests: TestRequirePermission_AbortsOnInvalidToken,
TestWhitelistIPMiddleware_AbortsOnFailure.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/frontend/apiserver/middleware/validatehandler.go

@@ -35,6 +35,7 @@ func ValidateHandler() gin.HandlerFunc {
 			if err != nil || !isValidBody {
 				controllers.ReturnError(ctx, common.INVALID_PARAMS_CODE,
 					common.ERROR_READ_REQUEST_BODY_MSG+": "+err.Error(), nil)
+				ctx.Abort()
 				return
 			}
 		}