What this PR does / why we need it:

Implements Phase 1 for #5411

Usage example:

# Current behavior (unchanged)
feast ui

# REST mode — mounts /api/v1/* routes from the REST registry server
feast ui --mode rest

# External REST — proxies /api/v1/* to a separate registry server
feast ui --mode rest-external --rest-api-url http://registry-host:6570/api/v1

# With proxy prefix
feast ui --mode rest --root_path /feast

Files Modified

• sdk/python/feast/cli/ui.py - Added two new CLI options:

--mode / -m: Accepts proto (default), rest, or rest-external
--rest-api-url: Required when using --mode=rest-external, specifies the external REST registry URL

Added validation: rest-external mode requires --rest-api-url

• sdk/python/feast/feature_store.py - Updated serve_ui() method:

Added mode and rest_api_url parameters, passed through to ui_server.start_server()

• sdk/python/feast/ui_server.py - Rewritten with three mode-specific setup functions:

Which issue(s) this PR fixes:

#5411

Checks

• I've made sure the tests are passing.
• My commits are signed off (git commit -s)
• My PR title follows conventional commits format

Testing Strategy

• Unit tests
• Integration tests
• Manual tests
• Testing is not required for this change