fix: make CLI flag values case-insensitive

[notgitika]

Description

CLI flag values like --framework strands fail because Zod enum validation is case-sensitive. This adds a matchEnumValue() utility that case-insensitively maps user input to the canonical enum value before validation, so strands, Strands, and STRANDS all work.

Normalization is applied to --framework, --language, --model-provider, and --build in both agentcore create and agentcore add agent/add mcp-tool commands.

Related Issue

Closes #261

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

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

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