feat(code-action): add quickfix to remove redundant parentheses

[takeokunn]

Summary

Add code action to remove redundant parentheses from Nix expressions.

Example Transformation:

Before	After
(1)	1
((1))	(1) then 1 (two separate actions)
(x)	x
({ })	{ }

Changes

File	Description
nixd/lib/Controller/CodeAction.cpp	Set isPreferred: true for redundant paren quickfix

Behavior

Action offered when:

• Cursor is on a parenthesized expression wrapping a "simple" form
• Simple forms: variable, integer, float, string, path, search path, paren, attribute set, list

Action NOT offered when:

• Inner expression requires parentheses for disambiguation (binary ops, if-then-else, lambda, let, with, etc.)

Handles removal:

• Removes both ( and ) with two TextEdit::mkRemoval edits
• Works with single-line and multiline expressions

Test Plan

• All 4 redundant-paren tests pass
• Build succeeds
• Tests cover:
  • basic.md: Remove parens around integer literal (1)
  • nested.md: Double-nested ((1)) produces two quickfix actions
  • multiline.md: Multiline paren removal across lines
  • needed-paren.md: Negative - (1 + 2) does NOT offer action

Related

• Issue: nixd: code actions #466
• Draft PR: [DRAFT] feat(code-action): add refactoring and quickfix actions for Nix code #755 (this implements Split 17)