fix(resilience): count resolved 5xx results against the provider breaker (#12360)

CircuitBreaker.execute() treated every resolved promise as a success, but handleChatCore() reports most upstream failures by resolving with { success: false, status: 5xx }. On the chat path that spurious _onSuccess() decayed failureCount right before the call site's _onFailure() for the same attempt, so a provider answering 503s indefinitely stayed CLOSED at failureCount: 1 and kept receiving traffic — the breaker was structurally unable to open. Combo dispatches hit the same cancellation through the shared per-provider breaker.

execute() now takes an optional per-call classifyResult; without it the resolved-means-success contract every throw-based caller relies on is unchanged. executeChatWithBreaker() passes ignore and the chat path accounts for the outcome exactly once where the request context lives, so a combo success is no longer counted twice.

Validated in a combined worktree with all 25 PRs of this batch boarded together: typecheck:core clean, 443/443 node-runner tests plus 14/14 vitest across every test file the batch touches, and check-changelog-integrity, check:cycles (418 files), check:provider-consistency (272 REGISTRY entries, 355 canonical providers), check:docs-counts, check:docs-sync (42 locales) and check-file-size all green.

Thanks @pacocartones.