Adds end-to-end CI coverage for `--use_v2_migration_resolver` via a new
job `installing_litellm_on_python_v2_migration_resolver`:
- Clones the pytest smoke path from `installing_litellm_on_python` but
uses a local Postgres sidecar instead of the shared DB to prevent
collisions with the v1 variant.
- Runs only the new `test_litellm_proxy_server_config_no_general_settings_v2_resolver`
which spawns the proxy with `--use_v2_migration_resolver` and smoke-tests
`/health/liveliness` and `/chat/completions`.
Refactors `test_basic_python_version.py`:
- Extracts the proxy spawn + smoke-test body into `_run_proxy_server_smoke_test`
so the v1 and v2 tests share the same code path.
- The existing `test_litellm_proxy_server_config_no_general_settings` is
now a thin wrapper that passes no extra args (v1 default, unchanged).
- Adds `..._v2_resolver` variant that passes `--use_v2_migration_resolver`.
The existing `installing_litellm_on_python` / `installing_litellm_on_python_3_13`
jobs filter out the v2 variant via `-k "not v2_resolver"` so they keep
running only against their shared DB, unchanged behavior.