fix: align REST message validation and SSE error format with JSON-RPC

[ez-lbz]

What changed

1. REST message/send and message:stream validate empty message parts

Problem: The REST endpoints (/message:send, /message:stream) bind the request with [FromBody], which does not validate that Message.Parts is non-empty. The JSON-RPC endpoint validates this in DeserializeAndValidate and rejects empty parts with InvalidParams. A REST client could send a message with no parts, which the server accepted — a REST/JSON-RPC validation gap.

Fix (src/A2A.AspNetCore/A2AHttpProcessor.cs):

• Added ValidateSendMessageRequest called at the start of SendMessageRestAsync and SendMessageStreamRest.
• Empty Message.Parts now throws A2AException("Message parts cannot be empty", A2AErrorCode.InvalidParams), which the existing exception handling maps to HTTP 400 — identical behavior to the JSON-RPC binding.

2. REST SSE error events use the structured JSON-RPC error shape

Problem: On a mid-stream error, the REST SSE writer (A2AEventStreamResult) emitted a hardcoded data: {"error":"An internal error occurred during streaming."} — losing the A2A error code, the exception message, and the structured data details. The JSON-RPC SSE stream (JsonRpcStreamedResult) returns a full code/message/data error object. Clients switching between transports got inconsistent error handling.

Fix (src/A2A.AspNetCore/A2AHttpProcessor.cs):

• A2AEventStreamResult now builds a structured error event: {"error":{"code":..,"message":..,"data":..}} via BuildErrorJson.
• A2AException error codes are preserved (code = the A2A JSON-RPC error code, e.g. -32001); unexpected exceptions fall back to -32603 (InternalError) with a generic message so internal details are never leaked.
• For A2A-specific error codes, data carries the same google.rpc.ErrorInfo array (@type, reason, domain) used by the JSON-RPC transport, so both transports produce consistent errors.

Testing

• dotnet test tests/A2A.AspNetCore.UnitTests --framework net8.0 — 95 passed, 0 failed (baseline 88, +7 new regression tests: empty-parts rejection for REST send and streaming send with HTTP 400 + error message; SSE error code/message/data preservation for A2AException; -32603 fallback without leaking generic exception messages; no error event on client disconnect; headers; valid data frames).
• dotnet test tests/A2A.UnitTests --framework net8.0 — 420 passed, 0 failed (unchanged).
• Behavior change: REST SSE error events now carry code/message/data instead of the hardcoded string. Clients parsing REST SSE errors should read the structured error object; non-error stream frames are unchanged. REST message/send/message:stream now reject requests with empty message parts (HTTP 400) instead of accepting them.