#1804 structured: one-string-field json_schema answered as plain text (opt-in), progress in structured keep-alives

Descripción

## Title
Structured output: a one-string-field schema answered as plain text (opt-in), and progress in structured keep-alives

## Summary
Split out of #1789 (which now only carries the Windows AMD images), as AGENTS.md asks: one change per PR, a new feature opt-in.

## What changed
- **`"structured_plain_string": true` (opt-in):** a `json_schema` that is one required string field, with an optional description/title and no enum, pattern or length limit, is answered as plain text, and the server builds the object around it.
  - Measured on a scanned rent roll (UD-IQ4_XS, temperature 0, a "write out every word on this page" prompt):

    | Asked for | Result |
    | --- | --- |
    | `{"text": ...}` | the table inside the string, then the escaped newline 256 times instead of the page's last lines and the closing quote |
    | plain text | the whole page in 165 tokens, ending on its own |

  - A model that writes the JSON object anyway is taken at its word, and that object is validated as before; an invalid one is still an error. Not with tools.
- **Structured SSE keep-alives** now say how far the answer is (`: keep-alive: structured output buffered until it is validated, N pieces (M characters) written so far`), so a long answer can be told from a stuck one. They are SSE comments, which clients ignore.
- **Docs:** `docs/DETAILS.md`.

## Extra Notes
- **Default path:** without the setting, `prepare_format` writes the same directive and `validated_json` validates as before.
- **The one change everyone gets:** the text of the structured-stream keep-alive comments, once content has been written. It is comment text only; data lines and the result are unchanged. Happy to put that behind the setting too if you prefer.
- **Tests:** `serve/test_structured.py` covers the wrap (streamed and not), JSON-anyway, off by default, which schemas qualify, and the progress notes. All `serve` and setup test files pass on `main` fb58e0d.

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