* fix(oauth): resolve re-auth deadlock and improve flow robustness
Problem: in-flight TCP server held port 18392 for up to 120 seconds.
Calling `auth()` again caused `bind()` to fail silently — the browser
opened but no listener could receive the OAuth callback. `_wipe()` on
exchange failure also destroyed credentials, forcing full re-setup.
Solution: `_active_close` at module scope cancels any in-flight server
when `auth()` or `clear_tokens()` is called. Binding is guarded with
`pcall`; the browser only opens after the server is listening. Swapped
`_wipe()` for `clear_tokens()` in `_exchange_code` to preserve
credentials on failure. Added `select_account` to `prompt` so Google
always shows the account picker on re-auth.
* test(oauth): isolate bundled-credentials fallback from real filesystem
Problem: `resolve_credentials` reads from `vim.fn.stdpath('data')`,
the real Neovim data dir. The test passed only because `_wipe()` was
incidentally deleting the user's credential file mid-run.
Solution: stub `oauth.load_json_file` for the duration of the test so
real credential files cannot interfere with the fallback assertion.
* ci: format
0cf3d29972