feat(failover): package-level default observer for transparently-built chains

The transparent comma-Parse path builds failover chains via NewFailoverModel
with no options, so defaultFailoverConfig() left the observer nil and observers
only fired when a caller passed WithFailoverObserver explicitly. Add a
package-level default observer (SetFailoverObserver / DefaultFailoverObserver),
guarded by the existing defaultsMu, and seed it in defaultFailoverConfig() so
chains built transparently still notify it. An explicit WithFailoverObserver
still overrides the default per-chain. mort sets this at boot to persist
failover events.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

v2/parse_test.go

@@ -12,10 +12,16 @@ import (
 // verify that Parse resolved to the correct model without network calls.
 type recordingProvider struct {
 	lastModel string
+	// err, when non-nil, is returned from Complete so failover tests can drive
+	// a comma-Parse'd chain through a failover decision. Defaults to nil (success).
+	err error
 }
 
 func (p *recordingProvider) Complete(_ context.Context, req provider.Request) (provider.Response, error) {
 	p.lastModel = req.Model
+	if p.err != nil {
+		return provider.Response{}, p.err
+	}
 	return provider.Response{Text: "ok"}, nil
 }