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.

tests/test_kicad_3rd_party_env_resolution.py

@@ -0,0 +1,75 @@
+"""
+Regression tests for ``${KICAD_3RD_PARTY}`` (no-version-prefix) URI resolution
+in both the symbol and footprint library managers.
+
+Background:
+    The Import-LIB-KiCad-Plugin (impart) documentation registers third-party
+    libraries using ``${KICAD_3RD_PARTY}`` without a KiCad-version prefix.
+    KiCad accepts both unprefixed and version-prefixed forms in lib-tables.
+
+    Prior to this fix the env-var dictionaries in ``_resolve_uri`` only handled
+    ``KICAD8/9/10_3RD_PARTY`` (analogous to ``KICAD_SYMBOL_DIR`` which *was*
+    in the dictionary without a prefix). Lib-table rows authored as
+    ``${KICAD_3RD_PARTY}/Foo.kicad_sym`` or ``${KICAD_3RD_PARTY}/Foo.pretty``
+    therefore failed to resolve and disappeared from MCP listings even though
+    KiCad's GUI showed them correctly.
+"""
+
+import sys
+from pathlib import Path
+
+import pytest
+
+sys.path.insert(0, str(Path(__file__).parent.parent / "python"))
+
+from commands.library import LibraryManager
+from commands.library_symbol import SymbolLibraryManager
+
+
+@pytest.mark.unit
+def test_symbol_manager_resolves_unprefixed_kicad_3rd_party(monkeypatch, tmp_path):
+    monkeypatch.setenv("KICAD_3RD_PARTY", str(tmp_path))
+    target = tmp_path / "EasyEDA.kicad_sym"
+    target.touch()
+
+    manager = SymbolLibraryManager.__new__(SymbolLibraryManager)
+    manager.project_path = None
+
+    resolved = manager._resolve_uri("${KICAD_3RD_PARTY}/EasyEDA.kicad_sym")
+    assert resolved == str(target)
+
+
+@pytest.mark.unit
+def test_library_manager_resolves_unprefixed_kicad_3rd_party(monkeypatch, tmp_path):
+    monkeypatch.setenv("KICAD_3RD_PARTY", str(tmp_path))
+    target_dir = tmp_path / "EasyEDA.pretty"
+    target_dir.mkdir()
+
+    manager = LibraryManager.__new__(LibraryManager)
+    manager.project_path = None
+
+    resolved = manager._resolve_uri("${KICAD_3RD_PARTY}/EasyEDA.pretty")
+    assert resolved == str(target_dir)
+
+
+@pytest.mark.unit
+def test_versioned_and_unprefixed_forms_both_work(monkeypatch, tmp_path):
+    """Both ``${KICAD10_3RD_PARTY}`` and ``${KICAD_3RD_PARTY}`` must resolve when
+    the corresponding env vars are set, even pointing at the same directory."""
+    monkeypatch.setenv("KICAD10_3RD_PARTY", str(tmp_path))
+    monkeypatch.setenv("KICAD_3RD_PARTY", str(tmp_path))
+
+    sym_target = tmp_path / "Mixed.kicad_sym"
+    sym_target.touch()
+    fp_target = tmp_path / "Mixed.pretty"
+    fp_target.mkdir()
+
+    sym_mgr = SymbolLibraryManager.__new__(SymbolLibraryManager)
+    sym_mgr.project_path = None
+    fp_mgr = LibraryManager.__new__(LibraryManager)
+    fp_mgr.project_path = None
+
+    assert sym_mgr._resolve_uri("${KICAD10_3RD_PARTY}/Mixed.kicad_sym") == str(sym_target)
+    assert sym_mgr._resolve_uri("${KICAD_3RD_PARTY}/Mixed.kicad_sym") == str(sym_target)
+    assert fp_mgr._resolve_uri("${KICAD10_3RD_PARTY}/Mixed.pretty") == str(fp_target)
+    assert fp_mgr._resolve_uri("${KICAD_3RD_PARTY}/Mixed.pretty") == str(fp_target)