Device flow now always clears stale credential state and re-fetches
clientID from MCP server, regardless of what previous login methods
(OAuth scan, PAT, etc.) left in app.json or runtime globals.
This replaces the previous Source-field approach with a simpler
defensive reset that is future-proof against new login methods.
Root cause: after OAuth login saved app.json without MCP source marker,
subsequent --device login reused the stale clientID without setting
clientIDFromMCP flag, causing exchangeCode() to use direct mode which
requires clientSecret.
Changes:
- Add resetCredentialState() to DeviceFlowProvider
- Login() always calls resetCredentialState() + FetchClientIDFromMCP()
- Remove AppConfig.Source field (no longer needed)
- Remove app_config_source_test.go (replaced by device_flow_reset_test.go)
- Add 5 tests covering OAuth→device, legacy→device, direct→device scenarios
Co-authored-by: 猷诺 <bicheng.ybc@alibaba-inc.com>
