feat(audience): surface server-side validation rejection reasons

[nattb8]

Summary

Ticket: SDK-684

Client-side validation (SDK-679) mirrors the server's rules, but the two will drift over time; today a rejection the server catches and the client doesn't is discarded silently once a flush completes. This adds a default, non-opt-in log per rejected message, and enriches the existing onError AudienceError with a typed Rejections list so integrators already using onError get structured data instead of hand-parsing the response. Covers both the 2xx-with-rejections and the 4xx-rejected response shapes, since the backend uses the same body for both.

This is lost data, not an advisory, so it logs at Log.Error (real Debug.LogError, not just a Warn-level text prefix) rather than Log.Warn. It's also aggregated into one call per flush instead of one per rejected message, since a batch can carry many rejections and separate Debug.LogError calls would flood the console.

	Before	After
Default visibility	HttpTransport only counted rejected; per-message detail was discarded	One Log.Error per flush listing every rejected messageId + field + reason, independent of onError
onError	AudienceError had no way to carry per-message detail at all	Adds Rejections: IReadOnlyList<MessageRejection>?, parsed from the same response
2xx rejection detection	Only surfaced when the body's numeric rejected count was > 0	Falls back to rejections.Count too, so a body where the two have drifted apart doesn't silently drop detail (4xx already didn't have this gap)

Test plan

• dotnet build and dotnet test pass: 389 passed, 1 skipped (pre-existing), 0 failed
• dotnet format --verify-no-changes clean
• New tests cover: Rejections populated on 2xx and 4xx, one aggregated Log.Error call fires with no onError wired, Log.Error uses ErrorWriter (not Writer) and fires regardless of Log.Enabled, malformed rejections in the body leaves it null instead of throwing, a 2xx body with rejected: 0 but a non-empty rejections array still fires onError
• Verified against a real rejected-message response from a running ingest API (local unit tests only here)

🤖 Generated with Claude Code

[github-actions]

Audience SDK — Build Size

Platform	SDK Size	Change
Android	0.39 / 20.00 MB	+0.03 MB	✅
Windows	0.17 / 20.00 MB	+0.01 MB	✅
iOS	8.61 / 20.00 MB	+0.09 MB	✅
macOS	1.00 / 20.00 MB	+0.10 MB	✅

SDK Size = build minus empty app. Change = vs baseline. Fails if any platform exceeds its absolute size limit.