Add AGUIStreamOptions.IncludeRawEvents

[halllo]

Closes #2297.

.NET servers attach the serialized originating ChatResponseUpdate as rawEvent to every TEXT_MESSAGE_*, TOOL_CALL_*, and REASONING_ENCRYPTED_VALUE event, with no way to opt out. On this repo's own Step* baselines that field is 66% of the wire. Nothing reads it, and neither the TS nor the Python server side emits it — so it is a .NET-only divergence from the canonical wire format.

This adds an init-only bool IncludeRawEvents (default true) to AGUIStreamOptions. The guard sits on the serialization rather than the assignment, so opting out also recovers the per-update CPU and allocations:

JsonElement? raw = options.IncludeRawEvents
    ? JsonSerializer.SerializeToElement(chatResponse, jsonSerializerOptions.GetTypeInfo(typeof(ChatResponseUpdate)))
    : null;

The default is true deliberately. false is arguably the parity-correct long-term default, but flipping it breaks anyone reading the field and moves every recorded baseline — better as its own deliberate change. This one is purely additive.

Every Create already took JsonElement?, so one internal signature widened (ReasoningMessageTracker.EmitEncryptedValue). Caller-supplied events (RawRepresentation or a Map* mapping) keep their own RawEvent. The event sequence is identical either way, so this is not a protocol change.

Tests

Six unit tests: default true; the field drops from text, tool-call, and REASONING_ENCRYPTED_VALUE events; caller-supplied rawEvent survives; identical event sequence either way. Plus RawEventSuppressionIntegrationTest, asserting the SSE body contains "rawEvent" only when the flag is on.

AGUI.Server.UnitTests 120/120 on net8.0/9.0/10.0, solution builds with 0 warnings, other suites pass. Two pre-existing failures are unaffected and reproduce on a clean checkout: cross-language needs a running TS server, and Step08_MultimodalMessagesTest reads an unfetched git-lfs pointer as its PNG.

Docs

New IncludeRawEvents section in hosting/extensibility.mdx, plus pointers from abstractions/events.mdx and sdks/dotnet/docs/architecture.md.