[ty] Add filtering option for mdtest runner (#21422)

## Summary

This change to the mdtest runner makes it easy to run on a subset of
tests/files. For example:

```
▶ uv run crates/ty_python_semantic/mdtest.py implicit
running 1 test
test mdtest__implicit_type_aliases ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 281 filtered out; finished in 0.83s

Ready to watch for changes...
```

Subsequent changes to either that test file or the Rust source code will
also only rerun the `implicit_type_aliases` test.

Multiple arguments can be provided, and filters can either be partial
file paths (`loops/for.md`, `loops/for`, `for`) or mangled test names
(`loops_for`):
```
▶ uv run crates/ty_python_semantic/mdtest.py implicit binary/union

running 2 tests
test mdtest__binary_unions ... ok
test mdtest__implicit_type_aliases ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 280 filtered out; finished in 0.85s

Ready to watch for changes...
```

## Test Plan

Tested it interactively for a while

crates/ty_python_semantic/mdtest.py

@@ -9,11 +9,12 @@
 
 from __future__ import annotations
 
+import argparse
 import json
 import os
 import subprocess
 from pathlib import Path
-from typing import Final, Literal, Never, assert_never
+from typing import Final, Literal, assert_never
 
 from rich.console import Console
 from watchfiles import Change, watch
@@ -32,10 +33,15 @@ MDTEST_DIR: Final = CRATE_ROOT / "resources" / "mdtest"
 class MDTestRunner:
     mdtest_executable: Path | None
     console: Console
+    filters: list[str]
 
-    def __init__(self) -> None:
+    def __init__(self, filters: list[str] | None = None) -> None:
         self.mdtest_executable = None
         self.console = Console()
+        self.filters = [
+            f.removesuffix(".md").replace("/", "_").replace("-", "_")
+            for f in (filters or [])
+        ]
 
     def _run_cargo_test(self, *, message_format: Literal["human", "json"]) -> str:
         return subprocess.check_output(
@@ -117,13 +123,16 @@ class MDTestRunner:
             check=False,
         )
 
-    def _run_mdtests_for_file(self, markdown_file: Path) -> None:
-        path_mangled = (
+    def _mangle_path(self, markdown_file: Path) -> str:
+        return (
             markdown_file.as_posix()
             .replace("/", "_")
             .replace("-", "_")
             .removesuffix(".md")
         )
+
+    def _run_mdtests_for_file(self, markdown_file: Path) -> None:
+        path_mangled = self._mangle_path(markdown_file)
         test_name = f"mdtest__{path_mangled}"
 
         output = self._run_mdtest(["--exact", test_name], capture_output=True)
@@ -165,9 +174,9 @@ class MDTestRunner:
 
             print(line)
 
-    def watch(self) -> Never:
+    def watch(self):
         self._recompile_tests("Compiling tests...", message_on_success=False)
-        self._run_mdtest()
+        self._run_mdtest(self.filters)
         self.console.print("[dim]Ready to watch for changes...[/dim]")
 
         for changes in watch(*DIRS_TO_WATCH):
@@ -214,12 +223,12 @@ class MDTestRunner:
 
             if rust_code_has_changed:
                 if self._recompile_tests("Rust code has changed, recompiling tests..."):
-                    self._run_mdtest()
+                    self._run_mdtest(self.filters)
             elif vendored_typeshed_has_changed:
                 if self._recompile_tests(
                     "Vendored typeshed has changed, recompiling tests..."
                 ):
-                    self._run_mdtest()
+                    self._run_mdtest(self.filters)
             elif new_md_files:
                 files = " ".join(file.as_posix() for file in new_md_files)
                 self._recompile_tests(
@@ -231,8 +240,19 @@ class MDTestRunner:
 
 
 def main() -> None:
+    parser = argparse.ArgumentParser(
+        description="A runner for Markdown-based tests for ty"
+    )
+    parser.add_argument(
+        "filters",
+        nargs="*",
+        help="Partial paths or mangled names, e.g., 'loops/for.md' or 'loops_for'",
+    )
+
+    args = parser.parse_args()
+
     try:
-        runner = MDTestRunner()
+        runner = MDTestRunner(filters=args.filters)
         runner.watch()
     except KeyboardInterrupt:
         print()