test(razorpaybilling): drive coverage 8.5% -> 100%

Every Portal method (SubscriptionID, CancelAtCycleEnd, CancelImmediately,
ListSubscriptionInvoices, PaymentUpdateURL, ChangePlan,
FetchSubscriptionDetails) plus the toInt64 / pickInvoiceTimestamp
helpers now exercise their happy path, every error branch (4xx/5xx
upstream, no-config, DB error, sql.ErrNoRows, malformed/missing fields),
and every callWithBreaker arm — including the open-state rejection
and the WithOnOpen callback on the singleton breaker.

The Razorpay REST surface is mocked via httptest.Server; no live API
calls. A one-line test seam in portal.go (newClientForPortal) lets
tests retarget the SDK's BaseURL at the mock; production callers go
through the original NewTimeoutClient unchanged.

Coverage: 8.5% -> 100.0% of statements (go test -cover).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mastermanas805

internal/razorpaybilling/portal.go

@@ -136,14 +136,21 @@ type Portal struct {
 	Cfg *config.Config
 }
 
+// newClientForPortal is the factory used by Portal.client(). It is a
+// package-level variable so unit tests in this package can install a
+// version that points its BaseURL at an httptest.Server mock of the
+// Razorpay API. Production code path is unchanged — the default is
+// NewTimeoutClient verbatim.
+var newClientForPortal = NewTimeoutClient
+
 func (p *Portal) client() (*razorpay.Client, error) {
 	if p.Cfg.RazorpayKeyID == "" || p.Cfg.RazorpayKeySecret == "" {
 		return nil, fmt.Errorf("billing not configured")
 	}
 	// P0-2: 30s HTTP timeout via ApplyHTTPTimeout — never the bare SDK
 	// default (10s) which is below Razorpay's documented p99 for
 	// subscription create.
-	return NewTimeoutClient(p.Cfg.RazorpayKeyID, p.Cfg.RazorpayKeySecret), nil
+	return newClientForPortal(p.Cfg.RazorpayKeyID, p.Cfg.RazorpayKeySecret), nil
 }
 
 // SubscriptionID returns the Razorpay subscription id stored on the team (stripe_customer_id column).