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 front: treat i2's NotSet sentinel as a signature default the same way as inspect.Parameter.empty ("required, no default").

• New front.util.param_default(param): returns param.default, or Parameter.empty when it is NotSet.
• Used everywhere front reads a signature default: InputBase.__post_init__ (value prefill), mk_input_element_specs (type inference, is_noneable), func_to_pyd_model_specs (pydantic default), _annotate_func_arguments (annotation from default type).

Why

i2mint/i2#88 gave FuncFactory signatures NotSet defaults and was reverted in i2mint/i2#89, because IntInputBase/FloatInputBase crashed on int(NotSet) and TextInputBase was prefilled with the text "NotSet". With this PR, front already behaves when #88 is re-applied.

No-op against the current i2: no current signature carries NotSet, so param_default(p) is p.default for every existing parameter.

i2 compatibility (decision)

Uses i2.is_not_set (added in i2mint/i2#91) when available, otherwise falls back on i2.deco.NotSet (present since 2023). So the i2 floor is not raised. Chosen over declaring a minimum i2 version because the consumer change must work before and after the i2 release, and raising a floor forces every front user to upgrade i2 for a no-op.

Tests

tests/test_not_set_defaults.py builds _foo's signature with NotSet defaults (using i2.deco.NotSet, so it works with any i2) and checks: no crash, no prefill, NotSet never reaches state; real defaults still prefill; input specs are identical to the plain signature's; pydantic specs mark the params required. 4 of these fail without the fix.

Gate

• pytest --doctest-modules front tests (scrap/examples ignored), py3.12: 71 passed, with i2 master (has is_not_set) and with i2 0.1.73 from PyPI (fallback path). Master: 63 passed.
• ruff check / ruff format --check on changed files: clean.
• Dependents (with this branch + i2 master), same as baseline: streamlitfront 16 passed (selenium-only modules skipped, not installed), opyratorfront 2 passed, extrude 3 passed.
• Hosted CI: see checks.

🤖 Generated with Claude Code

[thorwhalen]

Independent refute-review (sub-agent) could not refute the change: pydantic (incl. the field-rename path), crude dispatch and input specs all treat NotSet as required, with both i2 master and i2 0.1.73. Follow-ups applied in the latest commit:

• One test proved nothing. Worse, the fixture used Sig(_foo).ch_defaults(...)(_foo), which sets __signature__ on _foo itself, so the "plain" comparisons compared NotSet against NotSet. The fixture is now a separate function, input specs are keyed on the sentinel's type, and _annotate_func_arguments has a test. 6 of 8 tests now fail against master front.
• The fallback now imports i2.deco.NotSet at import time rather than on each call.

Accepted as-is (low): a crude-wrapped function called directly (not via the UI) with a missing arg now receives NotSet instead of raising TypeError. Python applies the signature default here, not front, and a real FuncFactory filters NotSet itself.
Out of scope here: streamlitfront's deprecated dispatch_funcs/SimplePageFunc path reads sig.defaults directly. It will be handled in a separate streamlitfront PR.

Gate re-run: 72 passed with i2 master and with i2 0.1.73.