fix: wire gateway-target CLI flags and default source to existing-endpoint

[aidandaly24]

Description

Fixes three bugs in the gateway-target CLI non-interactive path that made agentcore add gateway-target non-functional from the CLI:

Source code fixes:

• command.tsx: Wire source, type, and endpoint options through to the handler (were silently dropped, making --source existing-endpoint --endpoint <url> non-functional from CLI)
• validate.ts: Default source to 'existing-endpoint' when not provided, matching TUI behavior (only supported path currently)
• validate.ts: Reject --source create-new with a clear error message instead of falling through to unimplemented code
• validate.ts: Move outbound auth validation before source-specific branching so credential/OAuth checks apply to existing-endpoint targets (was unreachable due to early return)

After this fix, agentcore add gateway-target --name foo --endpoint https://example.com/mcp --gateway bar --json works as expected. Test coverage follows in a separate PR.

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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