fix: console buffer reuse after terminal open

[khaitranhq]

Summary

• Prevent Neogit from reusing a NeogitConsole buffer after nvim_open_term() has turned it into a terminal buffer.
• Recreate stale terminal-backed console buffers before applying non-terminal buffer options.

Problem

• Reopening the console could hit E474: Invalid argument when Neogit tried to set buftype = "nofile" on a buffer that was already buftype = "terminal".
• My environment:

NVIM v0.12.1
Build type: Release
LuaJIT 2.1.1774638290
Run "nvim -V1 -v" for more info

Debian 13.3 on WSL

Fix

• Add a guard in Buffer.create() that detects terminal-backed reused buffers.
• Delete and recreate the buffer before applying the standard Neogit buffer options.

Verification

• Ran make test
• All tests passed