Fix json_schema() dropping constraints for a Schema nested in a Schema

[gaoflow]

The bug

json_schema() renders a Schema nested directly inside another Schema as an empty {} schema, which matches anything — silently dropping the constraint and contradicting what validate() enforces.

from schema import Schema, Const

Schema(Const("fixed")).json_schema("http://x")
# -> {'$id': 'http://x', '$schema': '...draft-07...'}      # no "const"! accepts anything

Schema(Const("fixed")).validate("other")
# -> raises SchemaError (correctly rejects)

So the generated JSON schema disagrees with the validator on a representable construct. The same happens for any directly-nested schema, e.g. Schema(Schema(int)).json_schema(...) → {} instead of {"type": "integer"}.

Const("fixed").json_schema(...) and Schema("fixed").json_schema(...) both work — the loss only happens when the Schema/Const is wrapped in another Schema.

Root cause

In Schema.json_schema, _priority() classifies a Schema instance as VALIDATOR, and the only VALIDATOR branch in the generator handles Regex:

elif flavor == VALIDATOR and type(s) == Regex:
    ...
else:
    if flavor != DICT:
        return return_schema    # empty {}  <-- nested Schema lands here

validate() unwraps a nested schema (s = s.schema), but json_schema() never recurses into it, so every non-Regex nested Schema falls through to the no-op default.

Fix

Recurse into a nested Schema and merge its generated schema instead of dropping it:

if isinstance(s, Schema):
    return_schema.update(
        _json_schema(s, is_main_schema=False, allow_reference=allow_reference)
    )
elif flavor == TYPE:
    ...

Or/And/Regex are not Schema subclasses, so their existing branches are unaffected; only nested Schema instances (Const, Schema(Schema(...)), …) now expand.

Tests / verification

Added test_json_schema_const_and_nested_schema covering Const(literal) → const, Const(type) → type, Schema(Schema(int)) → type, and Const as a dict value. The new test fails on master and passes with the fix. Full suite incl. the README doctests is green (121 passed); ruff/ruff-format clean; no new mypy findings.