Problem: `_get_cloud_provider()` set `_cloud_provider_resolved = True`
before resolution. If credentials were briefly unavailable on the first
call (e.g. a managed Nous Portal token mid-refresh), the resolver pinned
the entire process to local mode forever, even after credentials
self-healed seconds later.
Root cause: bookkeeping was set up-front, so any code path that fell
through to `return _cached_cloud_provider` (config read failure, no
credentials yet, explicit-provider instantiation failure) committed the
transient `None` to the cache permanently.
Fix: invert the bookkeeping. `_cloud_provider_resolved = True` is now
set only when (a) the user explicitly chose `cloud_provider: local`, or
(b) a provider was successfully resolved. All transient `None` paths
return without poisoning the cache, so the next call retries. Explicit
provider instantiation failures now log at warning level with stack
trace so operators can diagnose them.
Tests: 5 new cases in tests/tools/test_browser_cloud_provider_cache.py
covering explicit local, successful resolution, no-credentials-yet,
config read failure, and explicit provider instantiation failure.
Stash-verify confirmed the 3 transient-None tests fail without the fix.
All 320 existing browser tests still green.
Closes#22324
