F#: completion auto-`open` insertion (FSAC parity) needs an unopened-symbol entity index

Gap

Ionide/FSAC offer auto-`open` on completion: accepting a completion for a type/value in a not-yet-opened namespace inserts the corresponding `open` directive (via `completionItem/resolve` → `additionalTextEdits`). SharpLsp's F# `completionItem/resolve` currently returns no edits.

This is tracked as item 11 in docs/plans/FSHARP-FSAC-PARITY-PLAN.md and as the `[FS-COMPLETION-RESOLVE]` follow-up in docs/plans/FSHARP-FEATURES-PLAN.md.

Why it's not a one-liner

The blocker is upstream of resolve: FSharpCompletion.getCompletions calls
checkResults.GetDeclarationListInfo(..., getAllEntities = fun () -> []) with an empty entity provider. With no entities, FCS never surfaces completion items for symbols in unopened namespaces, so DeclarationListItem.NamespaceToOpen is always None and there is nothing to auto-open.

Real auto-open requires:

1. An assembly-content / entity index (FSAC's AssemblyContentProvider) built from the project's referenced assemblies + the current file's symbols, fed into getAllEntities so unopened-namespace items appear in the list with a populated NamespaceToOpen.
2. A per-file resolve cache mapping the completion item index → the open-insertion TextEdit, computed at completion time with ParsedInput.FindNearestPointToInsertOpenDeclaration + ParsedInput.AdjustInsertionPoint. The Rust host's completionItem/resolve already maps AdditionalEdits → additionalTextEdits (src/semantic.rs), so once the sidecar returns edits it just works end-to-end.

Scope

Self-contained but non-trivial (entity index has its own caching/perf characteristics). Wire contract (CompletionResolveResultWire.AdditionalEdits) already exists.

Related parity work just landed: remove-unused-open / simplify-name code fixes ([FS-CODEFIX-*]) and F# workspace/symbol ([FS-WORKSPACE-SYMBOL]).

Start with FSharpCompletion.getCompletions and its getAllEntities provider, then inspect the AdditionalEdits mapping in src/semantic.rs and the referenced parity plans. Build the entity index and per-file resolve cache described in the issue, and verify that completionItem/resolve returns the open-insertion edit for symbols from unopened namespaces.