Add tree-sitter diagnostic provider for real-time error detection

[polarmutex]

Summary

Add a tree-sitter-based diagnostic provider to complement the existing beancount Python diagnostics, providing real-time error detection while editing.

Motivation

Currently, diagnostics are provided via beancount Python integration which may not catch all syntax errors in real-time. A tree-sitter diagnostic provider would offer:

• Real-time syntax error detection while typing
• Transaction balance validation - detect when current sum of transaction doesn't balance
• Structural validation - missing required fields, malformed entries
• Faster feedback - no need to invoke Python beancount for basic syntax issues

Proposed Features

Core Diagnostics

• Syntax errors from tree-sitter parsing
• Unbalanced transactions (sum != 0)
• Missing transaction descriptions
• Invalid account names (wrong format/structure)
• Date format validation
• Currency/commodity format validation

Advanced Diagnostics

• Detect incomplete postings (missing amounts)
• Flag suspicious patterns (unusual account combinations)
• Validate posting structure (account + amount pairs)
• Check for required metadata on certain transaction types

Implementation Plan

1. Create diagnostic provider module (crates/lsp/src/providers/diagnostics_treesitter.rs)
2. Integrate with existing diagnostic system in handlers.rs
3. Add tree-sitter query patterns for common error conditions
4. Implement transaction balance checking using tree-sitter AST traversal
5. Add configuration options to enable/disable specific diagnostic types

Benefits

• Immediate feedback without waiting for beancount processing
• Better developer experience while editing
• Complement existing Python-based diagnostics
• Foundation for more advanced real-time features

Related

• Existing diagnostics: python/bean_check.py
• Tree-sitter integration: already used for parsing
• LSP diagnostics handler: crates/lsp/src/handlers.rs