End-to-end regression coverage for #32243 that asserts every runtime
branch resolving an Anthropic endpoint returns
`api_mode == "anthropic_messages"`:
* `_resolve_explicit_runtime` — the path used when a Hermes
subcommand passes an explicit `--api-key` / `--base-url`. Pins
that a stale persisted `model.api_mode: chat_completions` from a
prior provider migration cannot override the anthropic pin.
* `_resolve_runtime_from_pool_entry` — the path triggered by
`hermes auth add anthropic --type oauth` (the exact flow from the
issue). Same stale-api_mode regression pinned here.
* `_try_resolve_from_custom_pool` — the user-defined
`providers:` / `custom_providers:` path that depends on the
URL detector fix landed in the prior commit. Asserts both the
detector fallback fires for `api.anthropic.com` and that an
explicit `api_mode_override` still wins (so users who DELIBERATELY
pointed a chat_completions transport at api.anthropic.com for
OpenAI-compat experiments aren't hijacked).
Co-locates the three contracts so a future refactor of one branch
cannot silently diverge from the others and re-introduce the
"out of extra usage" 400 on fresh OAuth Pro/Max credentials.
