s3/iam: reuse one request id per request (#8538)

* request_id: add shared request middleware

* s3err: preserve request ids in responses and logs

* iam: reuse request ids in XML responses

* sts: reuse request ids in XML responses

* request_id: drop legacy header fallback

* request_id: use AWS-style request id format

* iam: fix AWS-compatible XML format for ErrorResponse and field ordering

- ErrorResponse uses bare <RequestId> at root level instead of
  <ResponseMetadata> wrapper, matching the AWS IAM error response spec
- Move CommonResponse to last field in success response structs so
  <ResponseMetadata> serializes after result elements
- Add randomness to request ID generation to avoid collisions
- Add tests for XML ordering and ErrorResponse format

* iam: remove duplicate error_response_test.go

Test is already covered by responses_test.go.

* address PR review comments

- Guard against typed nil pointers in SetResponseRequestID before
  interface assertion (CodeRabbit)
- Use regexp instead of strings.Index in test helpers for extracting
  request IDs (Gemini)

* request_id: prevent spoofing, fix nil-error branch, thread reqID to error writers

- Ensure() now always generates a server-side ID, ignoring client-sent
  x-amz-request-id headers to prevent request ID spoofing. Uses a
  private context key (contextKey{}) instead of the header string.
- writeIamErrorResponse in both iamapi and embedded IAM now accepts
  reqID as a parameter instead of calling Ensure() internally, ensuring
  a single request ID per request lifecycle.
- The nil-iamError branch in writeIamErrorResponse now writes a 500
  Internal Server Error response instead of returning silently.
- Updated tests to set request IDs via context (not headers) and added
  tests for spoofing prevention and context reuse.

* sts: add request-id consistency assertions to ActionInBody tests

* test: update admin test to expect server-generated request IDs

The test previously sent a client x-amz-request-id header and expected
it echoed back. Since Ensure() now ignores client headers to prevent
spoofing, update the test to verify the server returns a non-empty
server-generated request ID instead.

* iam: add generic WithRequestID helper alongside reflection-based fallback

Add WithRequestID[T] that uses generics to take the address of a value
type, satisfying the pointer receiver on SetRequestId without reflection.

The existing SetResponseRequestID is kept for the two call sites that
operate on interface{} (from large action switches where the concrete
type varies at runtime). Generics cannot replace reflection there since
Go cannot infer type parameters from interface{}.

* Remove reflection and generics from request ID setting

Call SetRequestId directly on concrete response types in each switch
branch before boxing into interface{}, eliminating the need for
WithRequestID (generics) and SetResponseRequestID (reflection).

* iam: return pointer responses in action dispatch

* Fix IAM error handling consistency and ensure request IDs on all responses

- UpdateUser/CreatePolicy error branches: use writeIamErrorResponse instead
  of s3err.WriteErrorResponse to preserve IAM formatting and request ID
- ExecuteAction: accept reqID parameter and generate one if empty, ensuring
  every response carries a RequestId regardless of caller

* Clean up inline policies on DeleteUser and UpdateUser rename

DeleteUser: remove InlinePolicies[userName] from policy storage before
removing the identity, so policies are not orphaned.

UpdateUser: move InlinePolicies[userName] to InlinePolicies[newUserName]
when renaming, so GetUserPolicy/DeleteUserPolicy work under the new name.

Both operations persist the updated policies and return an error if
the storage write fails, preventing partial state.

test/volume_server/http/admin_test.go

@@ -23,8 +23,6 @@ func TestAdminStatusAndHealthz(t *testing.T) {
 	if err != nil {
 		t.Fatalf("create status request: %v", err)
 	}
-	statusReq.Header.Set(request_id.AmzRequestIDHeader, "test-request-id-1")
-
 	statusResp := framework.DoRequest(t, client, statusReq)
 	statusBody := framework.ReadAllAndClose(t, statusResp)
 
@@ -34,8 +32,8 @@ func TestAdminStatusAndHealthz(t *testing.T) {
 	if got := statusResp.Header.Get("Server"); !strings.Contains(got, "SeaweedFS Volume") {
 		t.Fatalf("expected /status Server header to contain SeaweedFS Volume, got %q", got)
 	}
-	if got := statusResp.Header.Get(request_id.AmzRequestIDHeader); got != "test-request-id-1" {
-		t.Fatalf("expected echoed request id, got %q", got)
+	if got := statusResp.Header.Get(request_id.AmzRequestIDHeader); got == "" {
+		t.Fatal("expected server-generated request id in response header")
 	}
 
 	var payload map[string]interface{}
@@ -49,7 +47,6 @@ func TestAdminStatusAndHealthz(t *testing.T) {
 	}
 
 	healthReq := mustNewRequest(t, http.MethodGet, cluster.VolumeAdminURL()+"/healthz")
-	healthReq.Header.Set(request_id.AmzRequestIDHeader, "test-request-id-2")
 	healthResp := framework.DoRequest(t, client, healthReq)
 	_ = framework.ReadAllAndClose(t, healthResp)
 	if healthResp.StatusCode != http.StatusOK {
@@ -58,8 +55,8 @@ func TestAdminStatusAndHealthz(t *testing.T) {
 	if got := healthResp.Header.Get("Server"); !strings.Contains(got, "SeaweedFS Volume") {
 		t.Fatalf("expected /healthz Server header to contain SeaweedFS Volume, got %q", got)
 	}
-	if got := healthResp.Header.Get(request_id.AmzRequestIDHeader); got != "test-request-id-2" {
-		t.Fatalf("expected /healthz echoed request id, got %q", got)
+	if got := healthResp.Header.Get(request_id.AmzRequestIDHeader); got == "" {
+		t.Fatal("expected /healthz server-generated request id in response header")
 	}
 
 	uiResp := framework.DoRequest(t, client, mustNewRequest(t, http.MethodGet, cluster.VolumeAdminURL()+"/ui/index.html"))