Incorrect completion inside interpolated string when prefix contains Unicode characters

Description

Completion inside a Ruby interpolated string behaves incorrectly when the string prefix contains Unicode characters (e.g., emoji).
The completion inserts duplicated/garbled text instead of replacing the identifier prefix correctly.

Ruby LSP Information

VS Code Version

1.130.0

Ruby LSP Extension Version

0.10.4

Ruby LSP Server Version

0.26.9

Ruby LSP Add-ons

• Standard Ruby (unknown)
• Ruby LSP RSpec (0.26.9)
• Ruby LSP Rails (0.4.8)
• RuboCop (1.84.2)

Ruby Version

3.4.8

Ruby Version Manager

rbenv

Installed Extensions

Ruby LSP Settings

{
  "formatter": "standard"
}

{
  "enabledFeatures": {
    "codeActions": true,
    "diagnostics": false,
    "documentHighlights": true,
    "documentLink": true,
    "documentSymbols": true,
    "foldingRanges": true,
    "formatting": true,
    "hover": true,
    "inlayHint": true,
    "onTypeFormatting": true,
    "selectionRanges": true,
    "semanticHighlighting": true,
    "completion": true,
    "codeLens": true,
    "definition": true,
    "workspaceSymbol": true,
    "signatureHelp": true,
    "typeHierarchy": true
  },
  "featuresConfiguration": {},
  "addonSettings": {},
  "rubyVersionManager": {
    "identifier": "auto"
  },
  "customRubyCommand": "",
  "formatter": "standard",
  "linters": null,
  "bundleGemfile": "",
  "testTimeout": 30,
  "pullDiagnosticsOn": "both",
  "useBundlerCompose": false,
  "bypassTypechecker": false,
  "rubyExecutablePath": "",
  "indexing": {},
  "erbSupport": true,
  "featureFlags": {},
  "sigOpacityLevel": "1"
}

Reproduction steps

1. Start the Ruby LSP using Visual Studio Code

2. Create a Ruby file

3. Write a minimal example

   test = "moo"
   "🐮 says #{te}"
   

4. Trigger completion on te and select test

Expected behavior

"🐮 says #{test}"

Actual behavior

"🐮 says #{tetest

Notice that the last " is overwritten

Notes

It looks like the replacement range/offset for completion in interpolated string content is calculated incorrectly when Unicode characters appear earlier in the same string. The same issue also occurs outside of interpolated string declarations if the source code line is prefixed with Unicode characters.

Start by reproducing the completion case in VS Code using the issue's Ruby example, then trace the completion replacement range and offset handling for Unicode-prefixed lines and interpolated strings. Done means selecting test produces "🐮 says #{test}" without overwriting the closing quote, including when Unicode appears outside interpolation.