fix(user-input-fold): match fenced code delimiters

[testikun]

Problem

Closes #334. The user-input fold parser treated any three-or-more backticks as a closing fence, so nested four-backtick blocks could close early; tilde fences were not recognized.

Fix

• Match closing fences to the opening fence character and minimum length.
• Support both backtick and tilde fenced blocks.
• Add regression coverage for nested and tilde fences.

Validation

• node --experimental-strip-types --test extensions/user-input-fold/index.test.ts
• 21 passed, 0 failed
• git diff --check