feat: show tool calls immediately while streaming

[ShrihariMahabal]

Tool cards used to appear only after the model finished streaming a call's arguments — a multi-second silent gap after a text block for calls with large arguments.

• Model layer emits a ToolCallBegin marker the moment a call's name is known mid-stream.
• The agent surfaces ToolStarted immediately (name only), then re-emits it with the full arguments once they finish streaming, before the tool runs — so arguments show during execution, not only with the result.
• Frontend tool_started upserts the card (create, then fill args).

[greptile-apps]

Confidence Score: 5/5

Safe to merge — the streaming path, upsert logic, and type annotations are all consistent and the announced set correctly prevents duplicate announcements.

The logic is tight: announced prevents double ToolCallBegin, the frontend upsert is correct for Vue 3 Proxy-based reactivity, and the test update validates the new emission. No broken contracts or data-loss paths introduced.

No files require special attention.

_{Reviews (2): Last reviewed commit: "test: expect ToolCallBegin marker in str..." | Re-trigger Greptile}