[editor-core] extract shared pieces for history-ot in editor (#36828)

* [editor-core] share the pieces the editor, real-time and history-v1 all need

Project-level history-OT has the browser, real-time and history-v1 all reason
about the same changes, so the rules they reason with move into the shared
library rather than being reimplemented on each side.

- `origin/editor_origin.js` plus `EDITOR_ORIGIN_KIND` and its schema entry: the
  origin an editor client's change carries, holding the editorId that scopes a
  change's timestamp.
- `change_identity.js`: how a submission is recognised when it comes back, so a
  resend of a change that already landed is deduplicated rather than applied
  twice.
- `rebase.js`: transforming a change onto concurrent ones, in the same order on
  both sides, which is what lets the server acknowledge a change with only the
  version it landed at.
- `text_file_defaults.js`: the doc-vs-file extension lists, now read from here
  by web's settings instead of being spelled out again.
- `blob_string_length.js`: the string length of a blob, used by history-v1's
  blob store and by web when it writes an uploaded file into history.
- `file_tree_diff.js`: file-tree operations to a diff, used by
  project-history's FileTreeDiffGenerator.
- `Blob.EMPTY_HASH`, so callers can answer for the empty blob without storing
  it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* [project-history] build the file-tree diff with the shared helper

FileTreeDiffGenerator's hand-rolled walk over add/edit/move/delete operations
becomes a call to `buildFileTreeDiff` from overleaf-editor-core, so the editor
and project-history read a file-tree change the same way. Adds the unit tests
the decaffeinated original never had.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* [web] read the editable-file lists from overleaf-editor-core

web's settings spelled out the text extensions and editable filenames that
decide whether a file becomes a doc, and every other service that has to make
that call carried its own copy. The lists now live in overleaf-editor-core and
web builds settings.textExtensions and settings.editableFilenames from them,
keeping the ADDITIONAL_TEXT_EXTENSIONS override for Server CE/Pro on top.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* [history-v1] read a blob's string length from overleaf-editor-core

Two copies of getStringLengthOfFile lived here, the one in recover_zip.js
carrying a TODO asking for exactly this. Both now import the shared
implementation, which the blob store keeps re-exporting for backupArchiver.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>
GitOrigin-RevId: 31948332f416f56848e3b0dfb5142a4c9f93f508

libraries/overleaf-editor-core/lib/schemas.js

@@ -258,6 +258,14 @@ const rawOrigin = z.union([
     version: z.number().int(),
     timestamp: z.iso.datetime(),
   }),
+  // An editor change that still carries the id of the editor instance it came
+  // from. Once a chunk is written the id is dropped from all but that editor's
+  // latest change, leaving a bare {kind: 'editor'} that the catch-all below
+  // accepts.
+  z.strictObject({
+    kind: z.literal('editor'),
+    editorId: z.uuid(),
+  }),
   z.strictObject({
     kind: z.string(),
   }),