feat(gateway): allow exact upstream endpoint URLs

[CaucherWang]

Summary

• add an append_endpoint_path upstream setting that defaults to enabled
• use the configured Base URL as the exact endpoint when path appending is disabled
• expose the setting in Studio with previews, validation, localization, documentation, and tests

Testing

• .venv/bin/pytest tests/gateway -q (705 passed)
• vitest run src/routes/gateway (361 passed)
• targeted ESLint and Prettier checks

[CaucherWang]

feature has been implemented in pr#6101