fix(composer): keep the caret inside the normalized document

CodeMirror collapses a CRLF pair into one line break, so the document is
shorter than the string it was given. The composer derived the caret from
the JS string length, which put it past the end of the document and made
dispatch throw `RangeError: Selection points outside of document`.

Because the exception fires before the transaction applies, the document
never updates, the un-normalized text stays in React state, and the draft
persists as-is: every later visit to the session restores it and crashes
again, with no way out from the UI.

Derive the caret from the change set instead, in the controlled writeback
and in the imperative insert/replace handles.

fixes #3013

# Conflicts:
#	CHANGELOG.md
#	packages/vscode/CHANGELOG.md

packages/ui/src/components/chat/composer/DOCUMENTATION.md

@@ -60,6 +60,15 @@ copy.
 exactly what gets sent, so nothing downstream serializes a rich document model
 back into a prompt.
 
+The document is not, however, the string it was given: CodeMirror normalizes
+line endings, so a `\r\n` pair becomes one break and the document ends up
+shorter than the inserted string. **Never derive a caret position from the
+length of text you are inserting** — a caret past the end makes `dispatch`
+throw, the transaction never applies, and the un-normalized text stays in React
+state to crash again on the next restore. Every edit that moves the caret goes
+through `replaceWithCaret` (`editor/documentEdits.ts`), which measures the
+change instead of the string.
+
 The composer previously painted a transparent `<textarea>` over a mirror
 `<div>`. That restricted highlighting to styles which do not change glyph
 advance width — colour, background, underline — because anything else made the