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refactor(argparse): use 'is' pattern for Some-or-noop match blocks (3 files)

Same shape as #3533 but in three other argparse files:

- `parser.mbt`: `match cmd.build_error { Some(err) => raise err; None => () }` and `match parent_matches.parsed_subcommand { Some(...) => big-block; None => () }`
- `parser_validate.mbt`: `match cmd.build_error { Some(err) => raise err; None => () }`
- `help_render.mbt`: `match arg.env { Some(env_name) => notes.push(...); None => () }`

Replaced with `if X is Some(y) { ... }` form.

Semantics-preserving: `moon test -p argparse` (225/225) passes and
`moon info` produces no `.mbti` change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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