fix: add AWS_IAM as a valid authorizer type for gateway commands

[alvarog2491]

Description

• agentcore add gateway --authorizer-type AWS_IAM was rejected with Invalid authorizer type. Use NONE or CUSTOM_JWT, while the interactive TUI accepted AWS_IAM successfully, an inconsistency introduced by a hardcoded allowlist in the CLI validator that had drifted from the Zod schema.
• Replaced the hardcoded list with GatewayAuthorizerTypeSchema.safeParse() so validation is driven by the schema and can never drift again.
• Updated help text, docs, and tests to reflect all three valid values (NONE, AWS_IAM, CUSTOM_JWT).

Related Issue

Closes #819

Documentation PR

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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