Read package.json workspaces through one helper: five readers disagree on non-string items, null and empty lists #1900

[agent] Filed by the scheduled architecture audit (audit-core). Register: register comment.

Kind: refactor. Register ID: A43.

Problem

The workspaces field of a package.json (the array form, or the object form's packages array) is parsed by hand in five places:

• hosted::governing_root::workspace_patterns, governing_root.rs L1209-L1225.`` It skips non-string items, returns None when the field is absent, and is already used by `npm_crawler::skipped_members_of`.
• npm_crawler::bun_workspace_pattern_members_sync, npm_crawler.rs L912-L937.``` A non-string item makes the whole read fail (pattern.as_str()?). An absent field or `null` reads as `Some([])`, and `!` patterns are dropped. The other caller in the same file hands `!` patterns on to `workspaces_include_as` (L124-L129).
• update::channel::declares_workspaces, channel.rs L390-L399. It checks only for a non-empty array, so a list made only of non-string items counts as workspaces here, while workspace_patterns would return an empty list for it.
• vendor/mod.rs, L237-L244. This is the yarn-classic workspace-risk warning.
• patch::user_patches, L93-L104.

All five parse the same shape. Each one decides on its own what a malformed field means.

Proposed change

Move workspace_patterns into the npm-family format layer, for example formats::json::workspace_patterns(&serde_json::Value) -> Option<Vec<String>>, next to the existing package.json helpers. Call it from all five sites. The Bun walk then keeps only the expansion step. The open question for the PR: should a non-string item fail closed everywhere, or be skipped everywhere? It must be one answer, and the Bun walk's fail-closed rule is the stricter one.

Size: 5 files, about −60/+25 lines.

Acceptance criteria

• Existing workspace tests pass: npm_crawler (Bun member and skipped-member tests), governing_root, update::channel and user_patches.
• A new table test of the helper covers: an absent field, null, an empty array, the object form, a non-string item and a ! pattern.