_try_anthropic() hard-failed (return None, None) when the anthropic
credential pool was present but had no selectable entry — e.g. the pooled
OAuth token expired and its refresh_token had gone stale, so
_select_pool_entry("anthropic") returned (True, None). This wedged every
auxiliary task routed to Anthropic (goal judge surfaced "no auxiliary
client configured") even when a perfectly valid ANTHROPIC_TOKEN /
credentials-file token was available. The main session stayed healthy
because it resolves the env token directly.
The openrouter path (_try_openrouter) and codex path already fall through
to their standalone credential on (True, None); anthropic was the only
provider that hard-failed. Make _try_anthropic fall through to
resolve_anthropic_token() on that branch so the three paths are symmetric:
a temporarily dead pool entry must not block auxiliary tasks when a valid
standalone credential exists.
Adds a regression test covering: (1) pool present + no entry + valid env
token -> client built from the env token, (2) pool present + no entry + no
resolvable token -> clean (None, None), (3) base_url defaults correctly
when falling through with pool_present=True.
