vscode: migrate settings.json writer to full hujson AST manipulation

Background

PR #688 added sqlcmd open vscode, which writes the user's settings.json to register a connection profile. The original implementation stripped JSONC comments and trailing commas, marshaled fresh JSON, and saved the user's original to a .sqlcmd-backup file. dlevy flagged that this clobbers the backup on every subsequent run (e.g. creating a second container loses the first backup) and silently destroys hand-authored comments.

PR #688 follow-up commit replaced the strip-and-marshal round trip with github.com/tailscale/hujson. We now parse the original AST and apply an RFC 6902 patch that touches only the two keys we own (mssql.connections, mssql.connectionGroups). Comments, trailing commas, and unrelated user keys round-trip untouched on replace operations, and the .sqlcmd-backup file is no longer needed.

Remaining limitation

hujson's Patch "add" operation (used when the document doesn't yet have an mssql.connections key) can blank a comment immediately adjacent to the insertion point. Unrelated keys are preserved; only a comment line at the insertion site is affected. This is a one-time event per settings.json (subsequent runs use the "replace" path, which fully preserves comments).

Additionally, createProfile / updateOrAddProfile in cmd/modern/root/open/vscode.go operate on map[string]interface{} for the connection array itself. This means per-connection comments inside mssql.connections[*] are not preserved when sqlcmd updates an existing profile.

Proposed work

Refactor cmd/modern/root/open/jsonc.go and the profile-construction helpers to operate directly on hujson.Value AST nodes instead of decoded maps. Specifically:

1. Walk the parsed root *hujson.Object to find mssql.connections and mssql.connectionGroups members, mutating their Value in place rather than going through a JSON patch.
2. For the array of profiles, locate the existing profile member by id and rewrite its *hujson.Object fields one at a time, leaving member ordering and surrounding extras intact.
3. When inserting a new profile or a new top-level key, set ObjectMember.Name.BeforeExtra explicitly so the preceding comment isn't absorbed by hujson's leading-comment extraction.

Verification

The round-trip test in cmd/modern/root/open/jsonc_test.go (TestApplyJSONCSettingsUpdates_PreservesComments) should be extended to cover the add-comment-adjacent case, and a new test should verify per-connection comments inside the mssql.connections array survive an update to an existing profile.

References

• PR: #688
• Related review thread: https://github.com/microsoft/go-sqlcmd/pull/688#discussion_r3325235606
• hujson patch semantics: https://github.com/tailscale/hujson/blob/main/patch.go

Start with cmd/modern/root/open/jsonc.go and cmd/modern/root/open/vscode.go to trace the current hujson parsing, profile construction, and update paths. Extend cmd/modern/root/open/jsonc_test.go, including TestApplyJSONCSettingsUpdates_PreservesComments, for add-adjacent and per-connection comments. Done means direct hujson AST updates preserve comments, ordering, and unrelated fields for new and existing profiles.