fix(bedrock-agentcore-alpha): runtime construct incorrectly forces default lifecycleConfiguration values (#36379)

### Issue # (if applicable)

Closes #36376 

### Reason for this change

The Runtime construct was incorrectly forcing default lifecycleConfiguration values even when users didn't specify them, causing unwanted lifecycle management settings to be applied and diverging from AWS Bedrock AgentCore API behavior where this configuration is completely optional.

### Description of changes
Modified to only set lifecycleConfiguration when explicitly provided by users, instead of forcing default values (idleRuntimeSessionTimeout: 900, maxLifetime: 28800)


### Describe any new or updated permissions being added
N/A



### Description of how you validated changes
* Updated and verified all existing unit tests pass
* Updated integration test snapshots that were affected by the removal of default lifecycle configuration

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

BREAKING CHANGE: Runtime constructs will no longer automatically include lifecycleConfiguration with default values when not explicitly specified by users.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: mazyu36

packages/@aws-cdk/aws-bedrock-agentcore-alpha/lib/runtime/runtime.ts

@@ -281,11 +281,14 @@ export class Runtime extends RuntimeBase {
       this.validateRequestHeaderConfiguration(props.requestHeaderConfiguration);
     }
 
-    this.lifecycleConfiguration = {
-      idleRuntimeSessionTimeout: props.lifecycleConfiguration?.idleRuntimeSessionTimeout ?? LIFECYCLE_MIN_TIMEOUT,
-      maxLifetime: props.lifecycleConfiguration?.maxLifetime ?? LIFECYCLE_MAX_LIFETIME,
-    };
-    this.validateLifecycleConfiguration(this.lifecycleConfiguration);
+    this.lifecycleConfiguration = props.lifecycleConfiguration ? {
+      idleRuntimeSessionTimeout: props.lifecycleConfiguration?.idleRuntimeSessionTimeout,
+      maxLifetime: props.lifecycleConfiguration?.maxLifetime,
+    } : undefined;
+
+    if (this.lifecycleConfiguration) {
+      this.validateLifecycleConfiguration(this.lifecycleConfiguration);
+    }
 
     if (props.executionRole) {
       this.role = props.executionRole;

packages/@aws-cdk/aws-bedrock-agentcore-alpha/lib/runtime/types.ts

@@ -44,15 +44,15 @@ export interface RequestHeaderConfiguration {
 export interface LifecycleConfiguration {
   /**
    * Timeout in seconds for idle runtime sessions. When a session remains idle for this duration,
-   * it will be automatically terminated. Default: 900 seconds (15 minutes).
-   * @default - 900 seconds (15 minutes)
+   * it will be automatically terminated.
+   * @default undefined - service default setting is 900 seconds (15 minutes)
    */
   readonly idleRuntimeSessionTimeout?: Duration;
 
   /**
    * Maximum lifetime for the instance in seconds. Once reached, instances will be automatically
-   * terminated and replaced. Default: 28800 seconds (8 hours).
-   * @default - 28800 seconds (8 hours)
+   * terminated and replaced.
+   * @default undefined - service default setting is 28800 seconds (8 hours)
    */
   readonly maxLifetime?: Duration;
 }

packages/@aws-cdk/aws-bedrock-agentcore-alpha/test/agentcore/runtime/integ.runtime-cognito.js.snapshot/aws-cdk-bedrock-agentcore-runtime-cognito.assets.json

@@ -355,10 +355,7 @@
       }
      }
     },
-    "LifecycleConfiguration": {
-     "IdleRuntimeSessionTimeout": 60,
-     "MaxLifetime": 28800
-    },
+    "LifecycleConfiguration": {},
     "NetworkConfiguration": {
      "NetworkMode": "PUBLIC"
     },

packages/@aws-cdk/aws-bedrock-agentcore-alpha/test/agentcore/runtime/integ.runtime-cognito.js.snapshot/aws-cdk-bedrock-agentcore-runtime-cognito.template.json

@@ -237,10 +237,7 @@
     },
     "AgentRuntimeName": "endpoint_test_runtime",
     "Description": "Runtime for endpoint integration test",
-    "LifecycleConfiguration": {
-     "IdleRuntimeSessionTimeout": 60,
-     "MaxLifetime": 28800
-    },
+    "LifecycleConfiguration": {},
     "NetworkConfiguration": {
      "NetworkMode": "PUBLIC"
     },