Return network failures, timeouts and bad payloads as errors

[milten89]

Fixes BACKLOG P0 item 4 (ADR-0002: "Failures that escape the result"). This is the last P0 item.

Changes

Failure	Before	After
HttpRequestException while sending	thrown	ServiceUnavailableError with the original exception (Exception, and InnerException of ToException())
HttpRequestException / IOException while reading the body	thrown	ServiceUnavailableError
Timeout (OperationCanceledException while the caller's token is not cancelled)	thrown	new RequestTimeoutError (Timeout, Exception) / RequestTimeoutException
Caller cancellation	thrown	still thrown (OperationCanceledException)
Empty payload (null)	UnknownError	SerializationError
Undecodable charset in Content-Type	thrown (InvalidOperationException)	SerializationError
"rates": null, [null] items	ArgumentException from mappers	SerializationError

• Both catch (Exception) blocks are gone; only specific exception types are caught.
• Mappers reject bad API data with an internal InvalidPayloadException. The new NbpPayload.Map helper (used at all 25 client call sites) returns it as a SerializationError, writes an Error log and records it on the span. Array mappers now return IReadOnlyList<T>.

Behaviour changes (pre-1.0, put them in the release notes)

• Body reads are now limited by HttpClient.Timeout. With ResponseHeadersRead, the timeout used to stop applying once the headers arrived. A short timeout combined with a large date range can now produce a RequestTimeoutError.
• Code that wrapped calls in try/catch (HttpRequestException / TaskCanceledException) will no longer see those exceptions. It must check the result, or use EnsureSuccess(), which throws ServiceUnavailableException / RequestTimeoutException.
• The empty-payload error is now SerializationError instead of UnknownError.
• ServiceUnavailableError gains an optional exception constructor parameter. Source stays compatible; binary compatibility breaks.

Tests

• Unit:
  • network failure while sending and while reading;
  • timeout while sending and while reading the body;
  • unknown charset;
  • a timeout while reading a 400 body (gives BadRequestError without the server message);
  • caller cancellation during that read (still throws);
  • NbpPayload, and the mappers throwing InvalidPayloadException.
• WireMock: connection failure (server stopped), timeout, malformed JSON, [null] item, and "rates": null for the currency and table clients.
• Real API: the manual tests (integrationTest.runsettings) pass, 54/54.

Verification

• dotnet build OpenUrzednik.slnx: 0 errors (only the existing NU1902 warning).
• dotnet test -f net10.0: Core 84/84, Nbp 454/454, IntegrationTests 47 passed / 7 skipped (manual tests).
• dotnet format --verify-no-changes: clean.
• reviewer agent: no blocking findings. I fixed all three should-fix points (charset, client-level rates: null tests, behaviour changes listed above) and most of the nits.

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