Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

The Diffbot LLM endpoint supports response_format, but the client had no way
to pass it: the payload in ask.py was hardcoded to model/messages/stream.

Adds response_format to ask() and a new ask_json() that returns a parsed
object, both sync and async, plus json_schema_format() to build the payload.
The CLI gains --schema, and --json now uses real structured output instead of
nudging the prompt and slicing between the first { and last }.

Server-side (see ../diffbot-llm) the schema is compiled to an EBNF grammar and
the final answer is constrained to it, so this is enforced decoding rather than
a request the model may ignore. Two rough edges are handled client-side:

- The schema must be nested at json_schema.schema. Anywhere else and the server
  returns 200 with the constraint silently dropped, so we reject that locally.
- The grammar permits a <think> block before the final answer, so ask_json
  strips think blocks and markdown fences before parsing.

ask_json without a schema deliberately sends a permissive {"type": "object"}
json_schema rather than {"type": "json_object"}. json_object gets no grammar
applied, and the RAG loop's internal tool call is itself a valid JSON object,
so it can be returned as the final answer -- reproducible with a system message
on some queries (5/5 on one, 0/12 without a system message). A guard raises
ValidationError if a tool call ever comes back.

Tested with 44 unit tests and 7 live tests against the real endpoint, covering
flat and nested schemas, streaming with a schema, async, and a regression test
for the system-message case.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

b64f6a1 regenerated the lock for the 3.9 floor before bumping the version in
pyproject.toml, so the lock kept recording 0.2.2. CI runs uv sync --frozen,
which skips the freshness check, so this never surfaced -- but it reappeared
as a dirty working tree after any uv command, and would fail uv lock --check.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>