#1805 setup: install jsonschema, so json_schema answers are checked against their schema

说明

## Title
setup: install jsonschema, so json_schema answers are checked against their schema

## Summary
`serve/structured.py` validates `json_schema` answers with `jsonschema` when it is installed. Setup never installed it, so on a normal install answers were only checked to be one JSON object, and the server said so once (`the Python package jsonschema is not installed`). 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
- **`requirements.txt`:** `jsonschema==4.26.0`, pinned with its dependencies (`attrs`, `jsonschema-specifications`, `referencing`, `rpds-py`, `typing_extensions`).
- **`setup.py`:** `PY_PACKAGES` and the two "packages installed" lines.
- **`tools/test_setup_pins.py`:** the pre-pin install test knows the new dependencies.
- **`docs/DETAILS.md`:** setup installs it now; an older install gets it with UPDATE.bat / update.sh.

## Extra Notes
The server still imports it lazily (`test_the_server_does_not_import_jsonschema_at_start` passes), and still works without it. All `serve` and setup test files pass on `main` fb58e0d.

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