fix(a2a-client): read 0.3-shaped file parts and report parts the SDK can't read

The 1.0 SDK ignores unknown part fields, so a file part shaped {kind:file, file:{bytes|uri,
name, mimeType}} arrived as an empty part and vanished. A2AWireCompat rewrites such parts
to {raw|url, filename, mediaType} in JSON and SSE responses before the SDK reads them, and
marks any part it still can't read so the result lists it with its field names.

docs/a2a.md

@@ -300,6 +300,11 @@ Files:
 - crm.tar.gz (320000000 bytes, application/gzip): not saved, over 209715200 bytes
 ```
 
+The SDK speaks protocol 1.0 and would silently drop file parts in other shapes, so every A2A response (JSON and
+SSE) first passes `A2AWireCompat`: the 0.3 shape `{"kind":"file","file":{"bytes"|"uri","name","mimeType"}}` (or those
+fields directly on the part) becomes `{"raw"|"url","filename","mediaType"}`, and a part it still can't read is listed
+as `- part: not saved, not understood (fields: ...)` and logged with its field names (never values).
+
 Saving a task again overwrites its folder's files, so `action=status` with the `taskId` of a finished task fetches
 its files again (as long as the remote agent still returns them). Watch deliveries (`[A2A result from …]`) carry the
 same list.