Add tabbed Find/Replace dialog in ted and implement Editor find/replace API

[Copilot]

This PR adds the initial Find/Replace UX in ted (as requested in the Edit menu) and introduces the corresponding Editor API surface to perform find navigation and replacements.

• Ted UX: Edit menu + shared dialog

  • Added _Find... and _Replace... as the first items in the Edit menu.
  • Both actions open the same FindReplaceDialog with Tabs:
    • Find tab: find-next
    • Replace tab: find-next, replace, replace-all
  • _Find... opens on the Find tab; _Replace... opens on the Replace tab.

• Editor API: find/replace operations

  • Added Editor.FindNext, Editor.FindPrevious, Editor.ReplaceNext, and Editor.ReplaceAll.
  • Search uses ordinal/ordinal-ignore-case matching with optional wrap behavior.
  • Match navigation updates selection/caret to the found range; replace APIs operate on current/next match.
  • Added guardrails around selection/document bounds in matching and dialog prefill paths.

• Coverage updates

  • Added unit coverage for next/previous find and replace-next/replace-all behavior.
  • Added integration coverage asserting Edit menu exposure of Find... and Replace....

// Editor API now available to ted dialog and other callers:
editor.FindNext("needle", matchCase: false, wrapAround: true);
editor.ReplaceNext("needle", "replacement", matchCase: false, wrapAround: true);
int count = editor.ReplaceAll("needle", "replacement", matchCase: true);