Add session policy support to IAM (#8338)

* Add session policy support to IAM

- Implement policy evaluation for session tokens in policy_engine.go
- Add session_policy field to session claims for tracking applied policies
- Update STS service to include session policies in token generation
- Add IAM integration tests for session policy validation
- Update IAM manager to support policy attachment to sessions
- Extend S3 API STS endpoint to handle session policy restrictions

* fix: optimize session policy evaluation and add documentation

* sts: add NormalizeSessionPolicy helper for inline session policies

* sts: support inline session policies for AssumeRoleWithWebIdentity and credential-based flows

* s3api: parse and normalize Policy parameter for STS HTTP handlers

* tests: add session policy unit tests and integration tests for inline policy downscoping

* tests: add s3tables STS inline policy integration

* iam: handle user principals and validate tokens

* sts: enforce inline session policy size limit

* tests: harden s3tables STS integration config

* iam: clarify principal policy resolution errors

* tests: improve STS integration endpoint selection

weed/iam/sts/session_policy.go

@@ -0,0 +1,35 @@
+package sts
+
+import (
+	"encoding/json"
+	"fmt"
+	"strings"
+
+	"github.com/seaweedfs/seaweedfs/weed/iam/policy"
+)
+
+// NormalizeSessionPolicy validates and normalizes inline session policy JSON.
+// It returns an empty string if the input is empty or whitespace.
+func NormalizeSessionPolicy(policyJSON string) (string, error) {
+	trimmed := strings.TrimSpace(policyJSON)
+	if trimmed == "" {
+		return "", nil
+	}
+	const maxSessionPolicySize = 2048
+	if len(trimmed) > maxSessionPolicySize {
+		return "", fmt.Errorf("session policy exceeds maximum size of %d characters", maxSessionPolicySize)
+	}
+
+	var policyDoc policy.PolicyDocument
+	if err := json.Unmarshal([]byte(trimmed), &policyDoc); err != nil {
+		return "", fmt.Errorf("invalid session policy JSON: %w", err)
+	}
+	if err := policy.ValidatePolicyDocument(&policyDoc); err != nil {
+		return "", fmt.Errorf("invalid session policy document: %w", err)
+	}
+	normalized, err := json.Marshal(&policyDoc)
+	if err != nil {
+		return "", fmt.Errorf("failed to normalize session policy: %w", err)
+	}
+	return string(normalized), nil
+}