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

[mazyu36]

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

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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

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

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Security Guardian Results	130 ran	130 passed

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[github-actions]

	Tests	Passed ✅	Skipped	Failed
Security Guardian Results with resolved templates	130 ran	130 passed

Test	Result
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