feat: add OAuth credential setup and gateway output parsing to TUI deploy flow

[aidandaly24]

Description

The CLI deploy command (agentcore deploy) already handles OAuth credential setup and gateway output parsing, but the TUI deploy flow (the interactive terminal UI) was missing both. This meant users deploying through the TUI wouldn't get their OAuth providers created or their gateway URLs persisted to deployed-state.json.

This PR brings the TUI deploy flow to parity with the CLI:

OAuth credential setup in TUI preflight. The useCdkPreflight hook now runs setupOAuth2Providers during the identity-setup phase, right after API key setup. OAuth credentials (provider ARN, client secret ARN, callback URL) are collected and passed through to the deploy flow.

Gateway output parsing in TUI deploy. After CDK deploy completes, useDeployFlow now reads the MCP spec to get gateway definitions, calls parseGatewayOutputs to extract gateway URLs from CloudFormation outputs, and passes both gateways and OAuth credentials to buildDeployedState so they're persisted correctly.

Without this change, users who deploy via the TUI would have working agents but broken gateway URLs and missing OAuth providers in their deployed state.

Related Issue

Part of the MCP Gateway Phase 1 integration (gateway-integration branch). Task 9b.

Type of Change

• Bug fix

Testing

• 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