IDLE: Fix devdanzin high benefit/effort issues

These 8 are "Lowest effort to confirm, highest cost if left alone — all silent".

35 — Format Paragraph silently deletes characters from every non-comment line of a selection (format.py:59)
36 — A literal % in any setting strips every open window's keybindings and leaves them off (config.py:48)
27 — Copy-with-prompts drops the last line whenever the selection ends at column 10, 20, 100... (pyshell.py:1034)
60 — Saving a file whose coding cookie cannot encode its text writes a BOM and keeps the cookie (iomenu.py:324-326)
51 — restore_file_breaks appends without clearing, so Save As carries the old file's breakpoints over (pyshell.py:268)
59 — _study1 has no f-string state, so a PEP 701 multi-line f-string is read as three statements (pyparse.py:245)
43 — Three keysyms shipped in every keyset cannot be spelled by the key-configuration dialog (config_key.py:15)
34 — WidgetRedirector swallows every TclError, so text.index() can never raise on any IDLE widget (redirector.py:116)