fix: resolve ${KICAD_3RD_PARTY} (no version prefix) in lib URI resolvers (#187)

The Import-LIB-KiCad-Plugin documentation registers third-party libraries
using `${KICAD_3RD_PARTY}` (without a KiCad-version prefix). KiCad accepts
both unprefixed and version-prefixed forms in lib-tables, and the analogous
`KICAD_SYMBOL_DIR` was already handled in `library_symbol.py`'s env-var
dictionary.

`KICAD_3RD_PARTY` was missing from both `_resolve_uri` env-var dictionaries
(in `python/commands/library.py` and `python/commands/library_symbol.py`),
so lib-table rows authored as `${KICAD_3RD_PARTY}/Foo.kicad_sym` or
`${KICAD_3RD_PARTY}/Foo.pretty` failed to substitute, were treated as
non-existent paths, and disappeared from `list_symbol_libraries` /
`list_libraries` results — even though KiCad's GUI showed them correctly.

Changes:
  - Add `KICAD_3RD_PARTY` to both `_resolve_uri` env-var dictionaries.
  - Add `KICAD_3RD_PARTY` fallback in
    `SymbolLibraryManager._find_3rd_party_dir`.
  - Refactor `LibraryManager._find_kicad_3rdparty_dir` to check all four
    env-var forms (KICAD10/9/8_3RD_PARTY + KICAD_3RD_PARTY) consistently.
  - Add regression tests in `tests/test_kicad_3rd_party_env_resolution.py`.

Reproduces with:
  - Set `KICAD_3RD_PARTY` env var in the MCP server's environment.
  - Register `(lib (name "Foo") (type "KiCad") (uri "\${KICAD_3RD_PARTY}/Foo.kicad_sym") ...)` in the global sym-lib-table.
  - Place a real `Foo.kicad_sym` at the resolved path.
  - Before: `list_symbol_libraries` does not return `Foo`.
  - After: `Foo` is listed.

python/commands/library.py

@@ -145,6 +145,7 @@ class LibraryManager:
             "KICAD10_3RD_PARTY": self._find_kicad_3rdparty_dir(),
             "KICAD9_3RD_PARTY": self._find_kicad_3rdparty_dir(),
             "KICAD8_3RD_PARTY": self._find_kicad_3rdparty_dir(),
+            "KICAD_3RD_PARTY": self._find_kicad_3rdparty_dir(),
         }
 
         # Project directory
@@ -205,10 +206,11 @@ class LibraryManager:
         import json
 
         # 1. Check shell environment variable first
-        if "KICAD9_3RD_PARTY" in os.environ:
-            path = os.environ["KICAD9_3RD_PARTY"]
-            if os.path.isdir(path):
-                return path
+        for var in ("KICAD10_3RD_PARTY", "KICAD9_3RD_PARTY", "KICAD8_3RD_PARTY", "KICAD_3RD_PARTY"):
+            if var in os.environ:
+                path = os.environ[var]
+                if os.path.isdir(path):
+                    return path
 
         # 2. Check kicad_common.json for user-defined variables
         kicad_common_paths = [