fix: add missing autoclosebracket keymap(paired bracket deletion)

[bajrangCoder]

Closes: #2462

[greptile-apps]

Greptile Summary

This PR adds the missing CodeMirror bracket-deletion keymap. The main changes are:

• Adds closeBracketsKeymap when autoCloseBrackets is enabled.
• Keeps the normal keymap path unchanged when auto-close is disabled.
• Extends editor tests to pass base extension options.
• Adds tests for paired bracket Backspace behavior.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
src/cm/baseExtensions.ts	Adds closeBracketsKeymap behind the existing autoCloseBrackets option.
src/test/editor.tests.js	Adds bracket Backspace tests and a trailing optional helper argument for base extension options.

_{Reviews (1): Last reviewed commit: "fix" | Re-trigger Greptile}