feat: add managedBy enum field to AgentCoreProjectSpec schema

[notgitika]

Description

Adds a top-level managedBy enum field to AgentCoreProjectSpecSchema. This field tracks which tool manages the project infrastructure (currently only CDK). Defined as z.enum(['CDK']).default('CDK') for backwards compatibility — existing agentcore.json files without the field parse correctly with the default value applied.

Changes:

• Add ManagedBySchema and ManagedBy type export to schema
• Add managedBy field to AgentCoreProjectSpecSchema after version
• Update all construction sites and test fixtures
• Update LLM-compacted schema
• Update snapshots

Related Issue

Closes #

Documentation PR

N/A

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Other (please describe):

Testing

How have you tested the change?

• I ran npm run test:unit and npm run test:integ
• I ran npm run typecheck
• I ran npm run lint
• If I modified src/assets/, I ran npm run test:update-snapshots and committed the updated snapshots

Additional testing:

• Verified backwards compatibility against existing test project in agentcore/testing/ (no managedBy field parses with default CDK)
• Added 3 schema validation tests: default applied when omitted, explicit CDK accepted, invalid values rejected

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

Note: CDK package has matching schema change in a separate PR against agentcore-l3-cdk-constructs.

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[aidandaly24]

lgtm approved