#1314 serve: keep unfinished tool calls visible as content

Descripción

Replaces #572 after the history cleanup. The original commit was cherry-picked onto the new main, with a follow-up for the Responses API.

When generation ends partway through an announced tool call, its buffered text is discarded. Return that text as content so clients can see the truncated call. Keep its streamed arguments incomplete. The Responses adapter also keeps function and custom calls marked incomplete when returning this text.

Added parser and HTTP regression tests for natural EOF and token limits. `python -m unittest serve.test_server.UnfinishedToolCall serve.test_responses.ToolRoundTrip -q` passes.

I used Claude/Codex to assist with preparing, checking and double-checking this change.

Co-authored-by: beatcollector <[email protected]>