fix: include request URL in NetworkException

[dobby-coder]

Summary

Adds the request URL to NetworkException so callers wrapping this SDK can tell PKG vs Cryptify failures (and which Cryptify phase) apart without spelunking through stack traces.

• NetworkException gains a Url property; constructor signature is now (int statusCode, string body, string url).
• Message format updated to HTTP {status} at {url}: {body} (e.g. HTTP 401 at https://pkg.postguard.eu/v2/irma/sign/key: Unauthorized).
• Both EnsureSuccessAsync helpers (PkgClient, CryptifyClient) read response.RequestMessage?.RequestUri — set automatically by HttpClient — and pass it through. No per-call-site plumbing needed; all five failure sites (FetchMpkJsonAsync, FetchSigningKeysAsync, InitUploadAsync, StoreChunkAsync, FinalizeUploadAsync) gain context for free.
• Adds NetworkExceptionTests (3 xUnit cases) covering the new property, message formatting, and an empty-body sanity case.

Closes #26

Test plan

• dotnet build — net8.0 + net10.0, no warnings
• dotnet test — 25/25 pass on net10.0 (net8.0 runtime not installed in this workspace; CI will exercise it)
• CI green on the PR

[dobby-coder]

LGTM — would approve, but GitHub blocks self-approval on bot-authored PRs.

Implementation matches the suggested fix in #26 one-for-one: Url property added, both EnsureSuccessAsync helpers (src/Api/PkgClient.cs:71, src/Api/CryptifyClient.cs:131) read response.RequestMessage?.RequestUri with a <unknown> fallback, and message format becomes HTTP {status} at {url}: {body}.

Checked:

• Correctness — all five failure sites (FetchMpkJsonAsync, FetchSigningKeysAsync, InitUploadAsync, StoreChunkAsync, FinalizeUploadAsync) pick up the URL transparently via HttpClient's auto-set RequestMessage.
• Security — none of the constructed URLs carry auth tokens or query strings; bearer/cryptifytoken live in headers. The UUID path segment in the Cryptify upload URLs is operationally fine to surface in exception messages.
• Null safety — ?. chain + ?? fallback covers a missing RequestMessage/RequestUri.
• Tests — three new xUnit cases (NetworkExceptionTests) cover property population, message formatting, and empty body; 25/25 pass on net10.0 locally. CI net8.0 + net10.0 both pass.
• API surface — constructor signature change is breaking in theory, but NetworkException is thrown by the SDK and not meant for consumer construction; pre-1.0 release-please flow handles this under fix:.
• Build clean, no warnings.

Ready to merge.