feat: Improve parser error messages for better user experience

[jade-codes]

Problem

Currently, pest parsing errors are displayed directly to users, which can be cryptic and unhelpful. For example:

ParseError { variant: Expected(keyword), positives: [keyword], negatives: [] }

This tells the user almost nothing about what went wrong or how to fix it.

Current State

• ParseError struct in core/parse_result.rs captures pest errors
• error_codes.rs defines error code constants (P001-P007) but they are not used for parser errors
• The LSP displays parse_error.variant directly as the message (see syntax/sysml/parser.rs:63)

Proposed Solution

1. Error Code Integration

Map pest error patterns to our defined error codes:

• P001 (PARSER_SYNTAX_ERROR) - General syntax errors
• P002 (PARSER_UNEXPECTED_TOKEN) - When pest reports unexpected input
• P003 (PARSER_EXPECTED_TOKEN) - When pest reports expected tokens
• P006 (PARSER_UNTERMINATED) - Unterminated strings/comments

2. Human-Readable Error Messages

Create a PestErrorTranslator that:

• Analyzes pest positives and negatives lists
• Maps grammar rule names to user-friendly terms
• Provides context-aware suggestions based on error location

3. Example Transformations

Pest Error	Current Display	Improved Display
Expected([keyword])	Expected(keyword)	P003: Expected a keyword
Expected([identifier])	Expected(identifier)	P003: Expected an identifier
Expected([;])	Expected(;)	P003: Missing semicolon at end of statement

4. Contextual Help

For common errors, provide hints:

• Missing semicolon: Did you forget a semicolon at the end?
• Unmatched brace: Check for missing closing brace
• Reserved word as identifier: keyword cannot be used as a name

5. Implementation Plan

1. Phase 1: Create parser/error_translator.rs module
2. Phase 2: Integrate with ParseError (add code and hint fields)
3. Phase 3: Context-aware improvements

Related

• Error codes defined in core/error_codes.rs
• Current parsing: syntax/sysml/parser.rs, syntax/kerml/parser.rs
• LSP diagnostics: syster-lsp/src/server/diagnostics.rs