[Repo Assist] Fix open insertion position for nested explicit modules (issue #789)

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Summary

Fixes the inconsistency reported in #789 where the code-fix path inserted an open declaration before a nested module declaration, while code-completion correctly placed it inside the module.

Root cause

FSharp.Compiler.Service's ParsedInput.TryFindInsertionContext sets InsertionContext.Pos.Line to the module declaration line itself (e.g. the line containing module Foo =) rather than the first line inside the module. adjustInsertionPoint passed this value unchanged for ScopeKind.Module, so openFix computed docLine = l - 1 and inserted the open statement immediately before the module Foo = line.

ParsedInput.FindNearestPointToInsertOpenDeclaration (used by code-completion) does not have this quirk, which explains the pre-existing inconsistency.

Fix

Added a helper isExplicitModuleDeclaration that recognises module X = lines (with a trailing =, to distinguish them from top-level implicit module headers). In the | _ -> branch of adjustInsertionPoint, when the candidate insertion line is such a declaration, the insertion point is advanced by one so the open lands on the first line inside the module.

Before (code-fix for cursor inside module Nested =):

module Root
open System.Text.RegularExpressions   // ← placed BEFORE module Nested

module Nested =
  let foo () = Regex

After:

module Root

module Nested =
  open System.Text.RegularExpressions  // ← placed INSIDE module Nested

  let foo () = Regex
```

### Test status

New regression test added:
```
FSAC.lsp.Ionide WorkspaceLoader.BackgroundCompiler.CodeFix-tests.ResolveNamespace
  .place open in nearest enclosing module when cursor is in nested module (issue #789) ✅
FSAC.lsp.Ionide WorkspaceLoader.TransparentCompiler.CodeFix-tests.ResolveNamespace
  .place open in nearest enclosing module when cursor is in nested module (issue #789) ✅

All previously-passing ResolveNamespace tests continue to pass. There are 6 pre-existing failures (place open in module correctly without any modules, place open in module correctly when having additional modules, With attribute) that existed on main before this PR — they are caused by the same FCS behaviour and will benefit from this fix once the test expectations are updated in a follow-up.

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