[JosephOladele]

This PR covers four SDK issues. Each one has its own commit.

Closes #1265
Closes #1266
Closes #1267
Closes #1268

#1266: TypeScript SDK auto-retry with exponential backoff

• New client options: maxRetries (default 2; the old retries option still works as a deprecated alias), timeout (per attempt, default 30s, 0 disables), retryInitialDelayMs and retryMaxDelayMs.
• Retries on 429, 502, 503 and 504, on network errors, and on timeouts. Backoff is exponential with full jitter.
• On 429 the SDK waits at least as long as Retry-After asks, whether it's given in seconds or as an HTTP date. If the wait would be longer than retryMaxDelayMs, it returns the 429 instead of hanging.
• Only idempotent requests are retried: GET/PUT/DELETE, plus POST/PATCH when an Idempotency-Key is sent. payments.create(params, { idempotencyKey }) and refunds.create({ idempotency_key }) now send that header. A POST to create a payment without a key is never retried, so a retry can't double-charge.
• Timeouts use AbortSignal.timeout. If the last attempt times out, the SDK throws FluxaPayError with status 408 and code REQUEST_TIMEOUT.
• New tests in sdk.test.ts: default and configured retry counts, each retryable status, no retry on 500, POST with and without an idempotency key, the Retry-After cap, Retry-After as an HTTP date, the backoff schedule, timeouts, and timeout: 0.

#1265: Go SDK payment link and polling helpers

• client.CreatePaymentLink(ctx, *CreatePaymentLinkParams) (*PaymentLink, error), plus client.PaymentLinks.Create/Get/List. Request and response structs have json tags for every field. Parameters are validated client-side first (errors wrap ErrInvalidParams), so bad input never makes a request.
• client.WaitForPayment(ctx, paymentID, opts ...PollOption) (*Payment, error), also available as client.Payments.WaitForPayment:
  • The first poll happens immediately; after that the interval backs off exponentially. Options: WithPollInterval, WithMaxPollInterval, WithPollBackoff, WithPollTimeout, WithSuccessStatuses, WithFailureStatuses, WithOnPoll.
  • Context cancellation and deadlines are honoured, including during in-flight HTTP calls.
  • Transient errors (429, 5xx, network) keep polling. Permanent errors such as 404 or 401 are returned immediately.
  • A failure status returns *PaymentStatusError, which matches errors.Is(err, ErrPaymentNotSuccessful). A timeout or cancellation returns *WaitTimeoutError, which unwraps to the context error and records the last status seen.
• httptest tests cover success, already-paid, failure statuses, custom statuses, context timeout, WithPollTimeout, cancellation, transient-error retries, permanent errors, validation and the backoff math.

#1268: Go SDK examples and quickstart

• examples/checkout_flow: creates a payment, prints checkout_url, waits with WaitForPayment, and exits with a distinct code for success, failure, or timeout/Ctrl+C.
• examples/webhook_server: a net/http server. It reads the raw body with a size limit, verifies the signature and the 300s timestamp window, dispatches on event type, and shuts down gracefully.
• examples/payment_links: creates a fixed-amount or open-amount link and lists active links.
• examples/README.md: setting environment variables on macOS/Linux and PowerShell, run steps for each example, a Gin snippet, and an error-handling cheat sheet. The SDK README is updated too.
• The examples use only the standard library, so go.mod gets no new dependencies.
• New go-sdk-ci.yml runs go vet, builds the examples, and runs go test -race on Go SDK changes.

#1267: Python SDK mock server tests

• tests/conftest.py holds canonical response fixtures (payment, payment link, merchant, refund, error bodies) and a respx router fixture.
• tests/test_mock_server.py covers:
  • merchants, payments, payment links, refunds and webhooks, on both the sync and async clients
  • a 400/401/404/500 error matrix across 10 endpoints, plus 403/410/422/429/503, non-JSON error bodies and network errors
  • that the API key, standard headers and custom headers are sent on every request
• SDK changes this needed (all backward compatible):
  • typed exceptions BadRequestError, AuthenticationError, PermissionDeniedError, NotFoundError, ConflictError, RateLimitError (with retry_after) and ServerError, all subclassing FluxaPayError; field errors are exposed on .errors
  • new client.payment_links and client.merchants resources
  • headers= and timeout= client options; custom headers can't override Authorization
  • refunds send idempotency_key as the Idempotency-Key header
• New python-sdk-ci.yml runs pytest on Python 3.9–3.12 for PRs that change the Python SDK.

All new and changed SDK code is covered by tests, and the existing tests are unchanged.

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