fix(a2a): send v1 file Part shape; tolerate v1 server-side

[HongmingWang-Rabbit]

Summary

Image-only chats surface Error: message contained no text content. because canvas posts the v0 file-part shape {kind:"file", file:{uri,name,mimeType}} that the workspace runtime's a2a-sdk v1 protobuf parser silently drops. v1 Part has fields [text, raw, url, data, metadata, filename, media_type] — there is no kind, no nested file. With ignore_unknown_fields=True, ParseDict drops both legacy keys, producing a fully-empty Part. No text, no extracted attachments → the executor's "no text content" guard fires.

Three coordinated changes:

1. canvas/ChatTab.tsx — outbound file parts now use the v1 flat shape {url, filename, mediaType} so the v1 parser populates Part fields instead of dropping them.
2. workspace/executor_helpers.py — extract_attached_files learns a v1 detection branch (non-empty part.url + filename + media_type) alongside the existing v0 RootModel and flat-file shapes. Defends every runtime mounting the OSS wheel against any client still on the v0 wire.
3. canvas/message-parser.ts — extractFilesFromTask tolerates the v1 shape on incoming agent responses, so file chips render in chat history regardless of which Part shape the runtime emits.

Repro:
```python
from a2a.types import a2a_pb2
from google.protobuf.json_format import ParseDict
v0 = {"kind":"file","file":{"uri":"workspace:/x.png","name":"x.png","mimeType":"image/png"}}
p = ParseDict(v0, a2a_pb2.Part(), ignore_unknown_fields=True)

All fields empty → silently dropped by extract_attached_files (pre-fix).

```

Test plan

• `workspace/tests/test_executor_helpers.py` — new v1-shape test + empty-Part defense (86 passed)
• `canvas/.../message-parser.test.ts` — new v1 protobuf flat-shape extraction test + URL-basename fallback (91 passed)
• `tsc --noEmit` clean across canvas
• After merge + cascade: send image-only chat in hongming Claude Code Agent workspace; expect agent to receive the image (manifest line in prompt) instead of "no text content" error

Surfaced 2026-05-01 in hongming workspace after the /workspace chown fix unblocked uploads.