Fix STS AssumeRole with POST body param (#8320)

* Fix STS AssumeRole with POST body param and add integration test

* Add STS integration test to CI workflow

* Address code review feedback: fix HPP vulnerability and style issues

* Refactor: address code review feedback

- Fix HTTP Parameter Pollution vulnerability in UnifiedPostHandler
- Refactor permission check logic for better readability
- Extract test helpers to testutil/docker.go to reduce duplication
- Clean up imports and simplify context setting

* Add SigV4-style test variant for AssumeRole POST body routing

- Added ActionInBodyWithSigV4Style test case to validate real-world scenario
- Test confirms routing works correctly for AWS SigV4-signed requests
- Addresses code review feedback about testing with SigV4 signatures

* Fix: always set identity in context when non-nil

- Ensure UnifiedPostHandler always calls SetIdentityInContext when identity is non-nil
- Only call SetIdentityNameInContext when identity.Name is non-empty
- This ensures downstream handlers (embeddedIam.DoActions) always have access to identity
- Addresses potential issue where empty identity.Name would skip context setting

test/s3tables/testutil/docker.go

@@ -0,0 +1,66 @@
+package testutil
+
+import (
+	"context"
+	"net"
+	"net/http"
+	"os/exec"
+	"testing"
+	"time"
+)
+
+func HasDocker() bool {
+	cmd := exec.Command("docker", "version")
+	return cmd.Run() == nil
+}
+
+func MustFreePortPair(t *testing.T, name string) (int, int) {
+	httpPort, grpcPort, err := findAvailablePortPair()
+	if err != nil {
+		t.Fatalf("Failed to get free port pair for %s: %v", name, err)
+	}
+	return httpPort, grpcPort
+}
+
+func findAvailablePortPair() (int, int, error) {
+	httpPort, err := GetFreePort()
+	if err != nil {
+		return 0, 0, err
+	}
+	grpcPort, err := GetFreePort()
+	if err != nil {
+		return 0, 0, err
+	}
+	return httpPort, grpcPort, nil
+}
+
+func GetFreePort() (int, error) {
+	listener, err := net.Listen("tcp", "0.0.0.0:0")
+	if err != nil {
+		return 0, err
+	}
+	defer listener.Close()
+	return listener.Addr().(*net.TCPAddr).Port, nil
+}
+
+func WaitForService(url string, timeout time.Duration) bool {
+	client := &http.Client{Timeout: 2 * time.Second}
+	ctx, cancel := context.WithTimeout(context.Background(), timeout)
+	defer cancel()
+
+	ticker := time.NewTicker(500 * time.Millisecond)
+	defer ticker.Stop()
+
+	for {
+		select {
+		case <-ctx.Done():
+			return false
+		case <-ticker.C:
+			resp, err := client.Get(url)
+			if err == nil {
+				resp.Body.Close()
+				return true
+			}
+		}
+	}
+}