Better diagnostics + query validation

[REASY]

Goal

Make GraphQLite errors actionable so clients don’t have to guess which syntax is unsupported.

Problems Observed

• Errors like “Failed to transform MATCH clause” are too generic.
• No line/column or token detail for parse errors.
• No way to validate queries before execution.

Proposed Changes (graphqlite)

• Add precise parse errors with line/column and expected tokens.
• Provide a validate(query) API that returns structured diagnostics.
• Include a dialect_version and supported feature list so clients can adapt.

Acceptance Criteria

• Parse errors show line/col + a clear reason.
• validate() returns errors without running the query.

Implementation Notes

• Expose parser errors through the Rust API as a structured type.
• Add tests for error messages on invalid syntax.