feat: Add local symbol library search and 3rd party library support (#25)

Adds comprehensive local KiCad symbol library search functionality and fixes KICAD9_3RD_PARTY environment variable resolution.

Features:
- Symbol library search by name, LCSC ID, description, manufacturer, MPN
- Support for 3rd party libraries installed via Plugin and Content Manager
- New MCP tools: search_symbols, list_symbol_libraries, get_symbol_info
- Enhanced library path resolution for KiCad 8 and 9

This enables users with locally installed JLCPCB libraries to search and use components directly.

Co-authored-by: l3wi <l3wi@users.noreply.github.com>

python/kicad_interface.py

@@ -212,6 +212,7 @@ try:
     from commands.connection_schematic import ConnectionManager
     from commands.library_schematic import LibraryManager as SchematicLibraryManager
     from commands.library import LibraryManager as FootprintLibraryManager, LibraryCommands
+    from commands.library_symbol import SymbolLibraryManager, SymbolLibraryCommands
     logger.info("Successfully imported all command handlers")
 except ImportError as e:
     logger.error(f"Failed to import command handlers: {e}")
@@ -258,6 +259,9 @@ class KiCADInterface:
         self.export_commands = ExportCommands(self.board)
         self.library_commands = LibraryCommands(self.footprint_library)
 
+        # Initialize symbol library manager (for searching local KiCad symbol libraries)
+        self.symbol_library_commands = SymbolLibraryCommands()
+
         # Schematic-related classes don't need board reference
         # as they operate directly on schematic files
         
@@ -323,6 +327,12 @@ class KiCADInterface:
             "list_library_footprints": self.library_commands.list_library_footprints,
             "get_footprint_info": self.library_commands.get_footprint_info,
 
+            # Symbol library commands (local KiCad symbol library search)
+            "list_symbol_libraries": self.symbol_library_commands.list_symbol_libraries,
+            "search_symbols": self.symbol_library_commands.search_symbols,
+            "list_library_symbols": self.symbol_library_commands.list_library_symbols,
+            "get_symbol_info": self.symbol_library_commands.get_symbol_info,
+
             # Schematic commands
             "create_schematic": self._handle_create_schematic,
             "load_schematic": self._handle_load_schematic,