Wrap PSR-18 transport failures in a TransportException

[loevgaard]

Part of #10 (finding 9).

Why

QuickpayException's docblock promises that catch (QuickpayException $e) nets all SDK-thrown exceptions — but a DNS failure, refused connection or timeout came straight through as the HTTP client's ClientExceptionInterface, so consumers either missed them or had to know PSR-18 internals to catch them.

What

• New TransportException (final, extends \RuntimeException) implements both QuickpayException and Psr\Http\Client\ClientExceptionInterface — so an existing catch (ClientExceptionInterface $e) still matches. It exposes getRequest() (with the SDK headers applied), isNetworkError() (previous is a NetworkExceptionInterface), and the original exception as getPrevious(). Message: The request could not be sent [GET https://api.quickpay.net/payments]: connection refused (query string stripped, like the other exceptions).
• Client::request() wraps the PSR-18 exception; getLastRequest() records the attempt and getLastResponse() is reset to null for it.
• ClientInterface @throws updated (TransportException instead of ClientExceptionInterface); QuickpayException docblock now true; README error-handling section covers it (with the retry-safety hint that pairs with findByOrderId() from Add PaymentsEndpoint::findByOrderId() and typed list filters (PaymentsQuery) #12).

BC note

Only code that catches NetworkExceptionInterface / RequestExceptionInterface specifically (rather than ClientExceptionInterface) would no longer match — it should check $e->getPrevious() (or isNetworkError()) instead. Flagging so you can decide; I think fulfilling the documented "one catch nets everything" contract is worth it a week after 1.0.0.

Tests: network vs request failures (a small in-test throwing PSR-18 client), message/URI sanitizing, last request/response bookkeeping, catch (QuickpayException) netting it, and the hierarchy test covers both interfaces.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.58%. Comparing base (c992a67) to head (771d2dd).

Additional details and impacted files

@@             Coverage Diff              @@
##                1.x      #18      +/-   ##
============================================
+ Coverage     98.51%   98.58%   +0.06%     
- Complexity      148      152       +4     
============================================
  Files            24       25       +1     
  Lines           404      423      +19     
============================================
+ Hits            398      417      +19     
  Misses            6        6              

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