feat: wire existing-endpoint flow for gateway targets

[aidandaly24]

Description

Wire the existing-endpoint flow for gateway targets. When a user selects "existing-endpoint" as the source, the CLI now routes to createExternalGatewayTarget instead of the scaffold-based createToolFromWizard. This enables connecting to external MCP servers via endpoint URL without scaffolding Lambda code.

Changes:

• Route existing-endpoint source to createExternalGatewayTarget in CLI actions
• Reject --host flag for existing-endpoint targets (host is only relevant for scaffolded targets)
• Remove host selection step from TUI wizard (not needed for existing-endpoint flow)
• Add unit tests for routing and validation

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

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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[jesseturner21]

lgtm