Apply Megaparsec best practices to parser error handling

[Unisay]

This PR applies Megaparsec best practices to improve the parser error handling implementation from PR #7489.

Summary of Improvements

1. Remove Redundant try Wrappers

• Issue: Keywords wrapped in try (symbol ...) despite symbol being auto-backtracking since Megaparsec 4.4.0 (current version: 9.7.0)
• Fix: Removed 10 redundant try wrappers from keyword and delimiter parsing
• Impact: Improved performance, clearer code, follows Megaparsec idioms

2. Fix Duplicate Entry Bug

• Issue: "constr" keyword appeared twice in choice list (lines 118 and 123)
• Fix: Removed duplicate entry
• Impact: Correctness fix

3. Add Strategic Labels for Better Error Messages

• Issue: No labels used, resulting in generic error messages
• Fix: Added two-level labeling with <?> operator
  • Delimiter level: "opening parenthesis '('", "closing bracket ']'"
  • Choice level: "term keyword (builtin, lam, ...)"
• Impact: Clearer, more helpful error messages for users

4. Replace fail with <?> in Type Parser

• Issue: Type parser used explicit fail instead of idiomatic <?>
• Fix: Replaced with label operator
• Impact: More idiomatic Megaparsec code, cleaner implementation

5. Refactor to Use between Combinator

• Issue: Manual delimiter handling with intermediate bindings
• Fix: Use standard between combinator
• Impact: More declarative, less error-prone, follows parser combinator patterns

6. Expand Test Coverage

• Added: 6 new error message tests
  • Missing closing parenthesis
  • Missing closing bracket
  • Missing builtin operand
  • Missing con operands
  • Invalid keyword
  • Bracket mismatch
• Refactor: Extracted testParseError helper to eliminate duplication
• Organized: Created "Error Messages" test group
• Impact: Comprehensive coverage, maintainable test code

[kwxm]

Note also #7391.