feat: Cypress test coverage enhancement for network failure cases

[akash-c-k]

network-failure-e2e-test.cy.ts — 8 tests

Why it matters
Previously, there was no test coverage for error handling. Recent fixes in PRs #1375, #1389, and #1340 were shipped without safeguards against regressions. This suite ensures those scenarios are now reliably tested.

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Test Coverage

Test	What it verifies
500 on confirm → error displayed	Confirms the SDK surfaces an error message when the payment confirmation API fails
402 decline → submit re-enabled	Ensures the submit button is re-enabled after a card decline (regression coverage for PR #1375)
Declined card → error message shown	Verifies that a real sandbox decline triggers the correct UI error message
In-flight request → button disabled	Confirms the submit button is disabled while a payment is processing
PM API 503 → graceful fallback	Ensures the SDK renders a fallback UI instead of breaking when the API returns 503
PM API 500 → graceful fallback	Same as above, but for 500 responses
API key header present on PM request	Verifies the publishable key is sent correctly and not truncated (regression for PR #1389)
Headers on confirm call are correct	Ensures required headers (Content-Type, api-key) are included (regression for PR #1340)