fix: shell-escape generator-produced suggestion replacements

[nickeddy]

Supersedes #11796, which was auto-closed when my fork was deleted (GitHub permanently closes a fork's PRs on fork deletion and they can't be reopened against a new fork). I didn't mean to close that - thought things were in a merged state. This is the same commit, rebased onto current master.

Suggestions produced by Generator::script (Rust generators registered via the command-signatures crate) and by v2 script generators flowed through From<...::Suggestion> for Suggestion impls that copied the raw value into replacement with no shell-escaping. Once the upstream command-signatures fix for git add <tab> lands, this gap becomes user-visible: selecting a suggestion for a path containing spaces inserts an unescaped string into the buffer, so the shell parses it as multiple tokens (git add new file test.csv instead of git add new\ file\ test.csv).

Apply ShellFamily::escape after the From conversion in both the legacy (engine/argument/legacy.rs) and v2 (engine/argument/v2.rs) generator pipelines so generator-backed suggestions match the escaping behavior the path completer (engine/path.rs) already provides for filesystem-backed completions.

Closes #11072

Refs #11072, warpdotdev/command-signatures#271.

Description

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Testing

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Current behavior:

After fixes:

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BUG-FIX: shell-escape generator-produced suggestion replacements

[oz-for-oss]

@nickeddy

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[oz-for-oss]

@nickeddy

Every PR must be linked to a same-repo issue before Oz can review it.

Next step: open or find a same-repo issue describing this change, then link it to this PR by adding Closes #123 to the PR description (or using the "Development" sidebar on GitHub). A maintainer will mark the issue ready-to-implement when it is ready. Once it is marked, comment /oz-review to re-trigger review.

See the contribution guidelines for the full readiness model.

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