fix(nushell): switch wrapper to @complete attribute on untyped rest

[max-sixty]

Per fdncred's recommendation in nushell/nushell#18128, replaces the parameter-level [...args: string@\"nu-complete wt\"] with a function-level @complete attribute on an untyped [...args] rest. The completer signature moves from [context: string] (with manual split row \" \" reconstruction) to [spans: list<string>], which nushell pre-tokenizes — net deletion of seven lines of fragile token reassembly.

Untyped rest is the form nushell routes through its normal external-argument handling, so once nushell/nushell#18131 ships in stable nu the wrapper will automatically benefit from --flag=\"value\" quote stripping and ~ expansion without further changes. Until then --flag=\"value\" still arrives at the binary with literal quotes — a pre-existing bug, not a regression introduced here. Supersedes #2437 (the body-level regex strip workaround).

Test plan

• cargo test --test integration --features shell-integration-tests — 1717 passed (including all four nu wrapper cases)
• pre-commit run --all-files — clean
• Smoke-tested the rendered template parses cleanly under nu 0.112.1
• Hand-verify subcommand completion at trailing space (e.g. wt switch <TAB>) in an interactive nu session — couldn't drive reedline via PTY in test, but fdncred's example in Parity with bare external tokenization for --wrapped / extern rest args (--flag="value") nushell/nushell#18128 implies spans is populated at the cursor position

Ref nushell/nushell#18128, nushell/nushell#18131