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

[mastermanas805]

Summary

• Adds internal/razorpaybilling/portal_coverage_test.go covering every Portal method, both pure helpers (toInt64, pickInvoiceTimestamp), and every callWithBreaker branch including the singleton's open-state rejection and WithOnOpen callback.
• One-line test seam in portal.go: newClientForPortal (defaults to NewTimeoutClient) lets tests retarget the SDK's BaseURL at an httptest.Server. Production callers go through the original constructor unchanged.
• All Razorpay REST shapes are mocked locally — no live API hits, no DB hits (sqlmock for SubscriptionID and ChangePlan persistence).

Coverage

• Baseline: 8.5%
• After: 100.0% of statements
• Per-func: every function at 100% (verified via go tool cover -func).

Test plan

• go test ./internal/razorpaybilling -coverprofile=cov.out -count=1 -> 100.0%
• go test ./internal/razorpaybilling -race -count=1 -> pass
• go vet ./internal/razorpaybilling/... -> clean
• go build ./internal/razorpaybilling/... -> clean
• CI gate (test + vet) green on the PR

Notes:

• The singleton-breaker test runs LAST in source order; it intentionally leaves the singleton open (the only way to cover the rejection branch). All preceding tests reset the singleton's consecutive-failure counter via t.Cleanup(resetBreaker) so ordering across the rest of the file is irrelevant.

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

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