fix(nix): gate matrix extra to Linux in [all] profile (#7461)

* fix(nix): gate matrix extra to Linux in [all] profile

matrix-nio[e2e] depends on python-olm which is upstream-broken on modern
macOS (Clang 21+, archived libolm). Previously the [matrix] extra was
completely excluded from [all], meaning NixOS users (who install via [all])
had no Matrix support at all.

Add a sys_platform == 'linux' marker so [all] pulls in [matrix] on Linux
(where python-olm builds fine) while still skipping it on macOS. This
fixes the NixOS setup path without breaking macOS installs.

Update the regression test to verify the Linux-gated marker is present
rather than just checking matrix is absent from [all].

Fixes #4594

* chore: regenerate uv.lock with matrix-on-linux in [all]

tests/test_project_metadata.py

@@ -11,12 +11,19 @@ def _load_optional_dependencies():
     return project["optional-dependencies"]
 
 
-def test_matrix_extra_exists_but_excluded_from_all():
+def test_matrix_extra_linux_only_in_all():
     """matrix-nio[e2e] depends on python-olm which is upstream-broken on modern
     macOS (archived libolm, C++ errors with Clang 21+).  The [matrix] extra is
-    kept for opt-in install but deliberately excluded from [all] so one broken
-    upstream dep doesn't nuke every other extra during ``hermes update``."""
+    included in [all] but gated to Linux via a platform marker so that
+    ``hermes update`` doesn't fail on macOS."""
     optional_dependencies = _load_optional_dependencies()
 
     assert "matrix" in optional_dependencies
+    # Must NOT be unconditional — python-olm has no macOS wheels.
     assert "hermes-agent[matrix]" not in optional_dependencies["all"]
+    # Must be present with a Linux platform marker.
+    linux_gated = [
+        dep for dep in optional_dependencies["all"]
+        if "matrix" in dep and "linux" in dep
+    ]
+    assert linux_gated, "expected hermes-agent[matrix] with sys_platform=='linux' marker in [all]"