[ty] Support --python=<symlink to executable> (#18827)

## Summary

Fixes https://github.com/astral-sh/ty/issues/640. If a user passes
`--python=<some-virtual-environment>/bin/python`, we must avoid
canonicalizing the path until we've traversed upwards to find the
`sys.prefix` directory (`<some-virtual-environment>`). On Unix systems,
`<sys.prefix>/bin/python` is often a symlink to a system interpreter; if
we resolve the symlink too easily then we'll add the system
interpreter's `site-packages` directory as a search path rather than the
virtual environment's directory.

## Test Plan

I added an integration test to
`crates/ty/tests/cli/python_environment.rs` which fails on `main`. I
also manually tested locally that running `cargo run -p ty check foo.py
--python=.venv/bin/python -vv` now prints this log to the terminal

```
2025-06-20 18:35:24.57702 DEBUG Resolved site-packages directories for this virtual environment are: SitePackagesPaths({"/Users/alexw/dev/ruff/.venv/lib/python3.13/site-packages"})
```

Whereas it previously resolved `site-packages` to my system
intallation's `site-packages` directory

crates/ty/tests/cli/main.rs

@@ -656,20 +656,46 @@ impl CliTest {
         Ok(())
     }
 
-    pub(crate) fn write_file(&self, path: impl AsRef<Path>, content: &str) -> anyhow::Result<()> {
-        let path = path.as_ref();
-        let path = self.project_dir.join(path);
-
+    fn ensure_parent_directory(path: &Path) -> anyhow::Result<()> {
         if let Some(parent) = path.parent() {
             std::fs::create_dir_all(parent)
                 .with_context(|| format!("Failed to create directory `{}`", parent.display()))?;
         }
+        Ok(())
+    }
+
+    pub(crate) fn write_file(&self, path: impl AsRef<Path>, content: &str) -> anyhow::Result<()> {
+        let path = path.as_ref();
+        let path = self.project_dir.join(path);
+
+        Self::ensure_parent_directory(&path)?;
+
         std::fs::write(&path, &*ruff_python_trivia::textwrap::dedent(content))
             .with_context(|| format!("Failed to write file `{path}`", path = path.display()))?;
 
         Ok(())
     }
 
+    #[cfg(unix)]
+    pub(crate) fn write_symlink(
+        &self,
+        original: impl AsRef<Path>,
+        link: impl AsRef<Path>,
+    ) -> anyhow::Result<()> {
+        let link = link.as_ref();
+        let link = self.project_dir.join(link);
+
+        let original = original.as_ref();
+        let original = self.project_dir.join(original);
+
+        Self::ensure_parent_directory(&link)?;
+
+        std::os::unix::fs::symlink(original, &link)
+            .with_context(|| format!("Failed to write symlink `{link}`", link = link.display()))?;
+
+        Ok(())
+    }
+
     pub(crate) fn root(&self) -> &Path {
         &self.project_dir
     }