Send non-text parts (files and structured data)

[alDuncanson]

Handler can only build text parts, so a user can never send a file or structured data. This is the largest remaining content gap and it is outbound-only.

Where it stands

An A2A Part carries one of four payloads plus filename and media_type:

Payload	Send	Receive
text	yes	yes
raw (inline bytes)	no	summarized
url (file by reference)	no	summarized
data (structured)	no	summarized

Receiving is in better shape than it looks: part_kind, part_file, and part_data in service.py already back previews in the Tasks and Artifacts panels and in the CLI. Extracting a received part in full is tracked separately.

The work

A2AService._build_user_message hardcodes parts=[Part(text=message_text)]. That is the root of it.

• CLI: --file PATH (repeatable) and --data JSON on message send and message stream. A file should become a raw part with filename and a sniffed media_type, or a url part when given a URL.
• TUI: a way to attach a file to the composer.
• Both should refuse a file that exceeds a sane inline limit and point at the url form instead.

Acceptance

• handler message send --url URL --text "review this" --file ./report.pdf sends a two-part message
• handler message send --url URL --data '{"k":"v"}' sends a data part
• The TUI can attach a file
• Round-trip tested against a real agent, not only mocks

Part of the A2A v1.0 parity work.

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_{Context: Handler was audited against the A2A v1.0 specification after the v0.2.0 release. Method coverage at the time: service 7/11, CLI 7/11, TUI 4/11 (after #98), MCP 6/11. The four methods missing everywhere are ListTasks, ListTaskPushNotificationConfigs, DeleteTaskPushNotificationConfig, and GetExtendedAgentCard. A deterministic streaming test agent lives outside the repo at ~/code/handler-streaming-agent — it supports ask (input-required), slow (cancellable), and fail, and its card sets capabilities.streaming = true, which ADK does not do by default.}