ArgParser: reject default attributes on non-Choice fields at generation time

[Smaug123]

Fixes #571.

Problem

[<ArgumentDefaultFunction>] and [<ArgumentDefaultEnvironmentVariable>] are only ever read inside the ChoiceType branch of createParseFunction. On any non-Choice field the generator accepted them, generated code without complaint, and silently dropped the default — the field stayed required at parse time. This is a fail-fast gap: an attribute that cannot be honoured should be rejected at generation time.

Defaults are intentionally surfaced through Choice<'a, 'a> so a successful parse can distinguish a user-supplied value (Choice1Of2) from a parser-filled default (Choice2Of2). A bare field cannot express "was this defaulted?", so honouring the default is impossible — but the generator didn't say so.

Fix

A syntactic guard in toParseSpec's per-field fold, before the record/union/positional dispatch. If a field carries a default attribute:

• positional → rejected (positional args are collected, not defaulted);
• non-positional, type is Choice<_,_> → accepted, unchanged (an unequal Choice<int,string> still hits the pre-existing "prove types equal" error);
• non-positional, any other type (scalar, flag DU, record, union) → rejected with a message pointing at the Choice<'a,'a> convention.

The now-unreachable positional default-rejection arm inside the positional getChoice callback is removed (the guard front-runs it).

Tests

Five new tests in TestArgParserRejection.fs, written first and observed failing before the guard:

1. the issue's exact repro (bare flag DU + ArgumentDefaultFunction);
2. sibling attribute on a bare scalar (ArgumentDefaultEnvironmentVariable);
3. default on a record-typed field;
4. default on a positional field;
5. positive control — a Choice<int,int> field with a default still generates.

Verification

• New rejection fixture: 38/38 pass (4 were failing before the guard).
• ConsumePlugin builds clean — every real Choice-typed default field still generates.
• Broader arg-parser consumer suite: 222/222 pass.

🤖 Generated with Claude Code