feat(code-action): add selection-based JSON to Nix conversion action

[takeokunn]

Summary

Add a new code action that converts selected JSON text to Nix expression syntax.
This is a selection-based action that works on arbitrary text within a Nix file, allowing users to quickly convert JSON data structures to their Nix equivalents.

Example:

{"foo": 1, "bar": true, "nested": {"key": "value"}}
→
{
  foo = 1;
  bar = true;
  nested = {
    key = "value";
  };
}

Changes

File	Description
nixd/lib/Controller/CodeAction.cpp	Add escapeNixString(), jsonToNix(), and addJsonToNixAction() functions
Positive Tests
json-to-nix.md	Basic test for simple JSON object conversion
json-to-nix-array.md	Test for JSON arrays
json-to-nix-nested.md	Test for nested structures with proper indentation
json-to-nix-numeric.md	Test for numeric types (integer and float)
json-to-nix-types.md	Test for all JSON types (null, boolean, string, number)
json-to-nix-escaping.md	Test for escape sequences in strings
json-to-nix-interpolation.md	Test for ${ interpolation escaping
json-to-nix-special-keys.md	Test for keys requiring quoting
Negative Tests
json-to-nix-invalid.md	Test that invalid JSON does NOT offer the action
json-to-nix-empty.md	Test that empty objects {} do NOT offer the action
json-to-nix-empty-array.md	Test that empty arrays [] do NOT offer the action
Edge Case Tests
json-to-nix-depth-limit.md	Test safety limit for deeply nested structures
json-to-nix-width-limit.md	Test safety limit for very wide arrays/objects

Behavior

Action offered when:

• User has selected text in a Nix file
• Selected text starts with { or [
• Selected text is valid JSON
• JSON is non-empty (has at least one element)

Explicitly excluded:

• Invalid JSON: No action offered
• Empty structures ({}, []): Already valid Nix, no transformation needed
• Text not starting with { or [: Quick rejection for performance

Safety limits:

• Depth limit: 100 levels of nesting (prevents stack overflow)
• Width limit: 10,000 elements per array/object (prevents memory issues)

Escape handling:

• " → "
• \ → \
• ${ → ${ (prevent Nix interpolation)
• \n, \r, \t → preserved escape sequences

Test Plan

• 13 regression tests covering positive, negative, and edge cases
• All existing tests pass
• Manual testing in editor

Related

• Issue: nixd: code actions #466
• Draft PR: [DRAFT] feat(code-action): add refactoring and quickfix actions for Nix code #755 (this implements Split 4)