Preserve noremap when restoring abbreviation mappings

[azusa0612]

Summary

• preserve noremap when restoring insert-mode mappings after Unicode abbreviation input
• assert that an existing non-recursive Lua mapping remains non-recursive

Why

Unicode abbreviation handling temporarily replaces <CR> and <Tab>, then recreates the previous mappings. The recreation preserved several mapping attributes, including desc, but dropped noremap.

That can turn a non-recursive completion fallback mapping into a recursive one. With blink.cmp and nvim-autopairs, pressing Enter after entering a Lean abbreviation can then re-enter the same fallback mapping until Neovim overflows its stack or hangs.

This completes the mapping restoration addressed in #367 and fixes a recurrence of the symptom reported in saghen/blink.cmp#441.

Validation

• just retest spec/abbreviations/abbreviations_spec.lua with unwrapped Neovim: 14 passed
• StyLua check passed
• Luacheck: 0 warnings / 0 errors
• Selene: 0 errors
• manual reproduction on Neovim 0.12.2 with blink.cmp 1.10.2 and nvim-autopairs 0.10.0: \or → ∨, type (, then press Enter inside (); the mapping is restored with noremap=1 and Enter inserts a newline normally