gh-151678: Add tests for tkinter.Canvas (GH-151683)

gh-151678: Add tests for tkinter.Canvas (GH-151683)

Cover previously-untested Canvas methods in CanvasTest:

* item creation and types, bbox, coordinate conversion, move/scale,
  find and addtag queries, tags, item configuration, stacking order,
  text-item editing, selection, focus, scan and postscript;
* the create_arc, create_oval, create_bitmap, create_image,
  create_text and create_window item creation methods, checking
  coordinates, default and explicit options, valid enumerations and
  rejection of invalid values;
* tag_bind() and tag_unbind(), checking the returned function id and
  binding script, querying bound sequences, the add parameter, event
  delivery to items via a tag, and removal of a single binding by id
  or all bindings for a sequence.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>