feat: add outbound auth wizard step with inline credential creation

[aidandaly24]

Description

When creating a gateway target through the TUI wizard, users previously had no way to configure outbound authentication. They'd have to manually edit mcp.json to add auth config after the
fact.

This PR adds an outbound auth step to the wizard flow. After selecting a gateway, users now see:

1. Auth type selection — No authorization or OAuth 2LO
2. Credential selection (OAuth only) — Create a new credential inline or pick an existing one from agentcore.json
3. Inline credential creation — For OAuth: name, client ID, masked client secret, discovery URL. Uses SecretInput for sensitive fields.

If "No authorization" is selected, the wizard skips straight to confirm. If credential creation fails (e.g. duplicate name), the sub-flow resets so the user can try again.

The confirm screen now shows the auth type and credential name in the review.

Note: API Key was removed from the outbound auth options as MCP server targets only support OAuth and NoAuth per the service documentation.

New wizard flows:

• create-new: name → source → language → gateway → outbound-auth → confirm
• existing-endpoint: name → source → endpoint → gateway → outbound-auth → confirm

This is the TUI layer on top of the operations added in the previous PR (Task 12a), which added createCredential OAuth support and CLI flags.

Related Issue

Part of the MCP Gateway Phase 1 integration (gateway-integration branch). Task 12b.

Type of Change

• New feature

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

No new tests — this is TUI wiring over the operations layer tested in Task 12a. The credential creation, validation, and config writing are all covered by existing tests.

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

[notgitika]

LGTM