fix: make the schema codegen host-independent

The generated modules are checked in, and `make gen-check` asserts that regenerating
them reproduces the committed artifact. Two host dependencies broke that:

- `gen_schema.py:179` and `gen_meta.py:59` interpolate a `Path` into the generated
  header, so the path uses `os.sep` and a non-POSIX host emits `schema\schema.json`
  where the committed artifact says `schema/schema.json`.
- both generators run ruff via `subprocess.run(..., text=True)` without an encoding,
  so the child's UTF-8 output is decoded with `locale.getpreferredencoding(False)`.
  When ruff fails, the decode fails first and the message becomes "stream did not
  contain valid UTF-8", discarding the actual reason -- and hiding the header
  mismatch, since that error is raised before the artifact comparison.

`as_posix()` emits `/` on every host, and `encoding="utf-8"` is what both files
already pass to every read_text/write_text they perform.

tests/test_gen_all.py goes from 1 failed / 20 passed to 21 passed; reverting only
these two scripts restores the failure.

Author: PerryLink

scripts/gen_meta.py

@@ -56,7 +56,7 @@ def render_meta(*, protocol_version: int = 1) -> str:
     client_methods = data.get("clientMethods", {})
     protocol_methods = data.get("protocolMethods")
     version = data.get("version", 1)
-    header_lines = [f"# Generated from {meta_json.relative_to(ROOT)}. Do not edit by hand."]
+    header_lines = [f"# Generated from {meta_json.relative_to(ROOT).as_posix()}. Do not edit by hand."]
     if version_file.exists():
         ref = version_file.read_text("utf-8").strip()
         if ref:
@@ -74,6 +74,7 @@ def render_meta(*, protocol_version: int = 1) -> str:
         [sys.executable, "-m", "ruff", "format", "--stdin-filename", str(out_py), "-"],
         input=source,
         text=True,
+        encoding="utf-8",
         capture_output=True,
         check=False,
         cwd=ROOT,

scripts/gen_schema.py

@@ -176,7 +176,7 @@ def _deserialize_field_specs(definition: dict[str, Any]) -> tuple[list[str], lis
 
 
 def _build_header(schema_json: Path, version_file: Path) -> str:
-    lines = [f"# Generated from {schema_json.relative_to(ROOT)}. Do not edit by hand."]
+    lines = [f"# Generated from {schema_json.relative_to(ROOT).as_posix()}. Do not edit by hand."]
     if version_file.exists() and (ref := version_file.read_text(encoding="utf-8").strip()):
         lines.append(f"# Schema ref: {ref}")
     return "\n".join(lines)
@@ -192,6 +192,7 @@ def _format_python(source: str, schema_out: Path) -> str:
             [sys.executable, "-m", "ruff", *arguments, "--stdin-filename", str(schema_out), "-"],
             input=source,
             text=True,
+            encoding="utf-8",
             capture_output=True,
             check=False,
             cwd=ROOT,