fix: treat i2's NotSet signature default as required / no default

[thorwhalen]

What

Step 2 of the re-land plan in i2mint/i2#48, for py2http: treat i2's NotSet sentinel as a signature default the same way as inspect.Parameter.empty ("required, no default").

• New py2http.schema_tools.param_default(param): returns param.default, or Parameter.empty when it is NotSet.
• Used in mk_input_schema_from_func (OpenAPI request schema: default, required, type inference) and ParamsSpecifier.from_func. The latter also loses a dead loop whose result was immediately overwritten.

Why

i2mint/i2#88 gave FuncFactory signatures NotSet defaults and was reverted in i2mint/i2#89. One reason was that mk_routes_and_openapi_specs([factory]) then produced a spec that json.dumps refused ("Sentinel is not JSON serializable"), with the params no longer required. With this PR, the spec for a NotSet-defaulted signature is identical to the plain one.

Decision: NotSet means required. The plan left open whether such params should be required or optional in the schema. I chose required: it keeps py2http's output for a FuncFactory exactly as it is today, before and after #88. It also matches what NotSet stands for: the wrapped function will need the value by the time it is called. Making them optional would be a separate, deliberate API change.

No-op against the current i2: no current signature carries NotSet.

i2 compatibility (decision)

Uses i2.is_not_set (added in i2mint/i2#91, i2 0.1.74) when available, otherwise falls back on i2.deco.NotSet (present since 2023). The i2 requirement is not changed.

Tests

py2http/tests/test_not_set_defaults.py: the input schema and the whole OpenAPI spec for a NotSet-defaulted foo equal those of the plain foo, the spec JSON-serializes, and ParamsSpecifier uses its _dflt_default. There is also a doctest in mk_input_schema_from_func. 3 of the 4 tests fail without the fix (the 4th checks the fixture).

Gate

• New tests + schema_tools/decorators doctests: 15 passed with i2 master (native is_not_set) and with i2 0.1.73 from PyPI (fallback path).
• Full pytest --doctest-modules py2http (py3.12, scrap/examples ignored): 26 passed, 1 failed, 2 errors. Master gives 21 passed with the same 1 failure and 2 errors, all pre-existing: the service.mk_app doctest, and tests/simple_run_process_test.py calling http2py.mk_request_func_from_openapi_spec with a stale signature. Hosted CI on master has been red since 2025-08 for pre-existing reasons.
• Dependents of py2http: extrude (tested below); peruse is not on this box.
• Hosted CI: see checks.

🤖 Generated with Claude Code

[thorwhalen]

Independent refute-review (sub-agent): no blocking defects. It exercised a real FuncFactory whose __signature__ carries NotSet defaults (what #88 does): the spec JSON-serializes, has no NotSet, equals the plain factory's spec and keeps required: ['a','b']. The same holds with i2 master and with i2 0.1.73. It also ran a bottle app built with mk_app through WSGI: responses are identical with and without the change. Nothing in py2http fills missing args from signature defaults, so NotSet cannot reach the function or a response.

Follow-ups applied:

• Added a test with a real FuncFactory (not just a plain function). 4 of 5 tests fail against master's source.
• Commented that x-py-param (only with include_func_params=True, which nothing in the fleet uses) is the raw Parameter and may still carry NotSet. It was never JSON-able anyway.

Noted, not changed:

• Switching param.default != Parameter.empty to is not also fixes defaults with an exotic __eq__ (e.g. numpy arrays), which used to raise here.
• required is a spec-level fact only. validate_input does not enforce top-level required, so a missing arg gives a 500, not a 400. This is pre-existing and unchanged.
• A real FuncFactory has no __name__, so it cannot be routed without setting one. This is pre-existing and unchanged.