[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/python_environment.rs

@@ -292,6 +292,59 @@ fn python_version_inferred_from_system_installation() -> anyhow::Result<()> {
     Ok(())
 }
 
+/// On Unix systems, it's common for a Python installation at `.venv/bin/python` to only be a symlink
+/// to a system Python installation. We must be careful not to resolve the symlink too soon!
+/// If we do, we will incorrectly add the system installation's `site-packages` as a search path,
+/// when we should be adding the virtual environment's `site-packages` directory as a search path instead.
+#[cfg(unix)]
+#[test]
+fn python_argument_points_to_symlinked_executable() -> anyhow::Result<()> {
+    let case = CliTest::with_files([
+        (
+            "system-installation/lib/python3.13/site-packages/foo.py",
+            "",
+        ),
+        ("system-installation/bin/python", ""),
+        (
+            "strange-venv-location/lib/python3.13/site-packages/bar.py",
+            "",
+        ),
+        (
+            "test.py",
+            "\
+import foo
+import bar",
+        ),
+    ])?;
+
+    case.write_symlink(
+        "system-installation/bin/python",
+        "strange-venv-location/bin/python",
+    )?;
+
+    assert_cmd_snapshot!(case.command().arg("--python").arg("strange-venv-location/bin/python"), @r"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    error[unresolved-import]: Cannot resolve imported module `foo`
+     --> test.py:1:8
+      |
+    1 | import foo
+      |        ^^^
+    2 | import bar
+      |
+    info: make sure your Python environment is properly configured: https://github.com/astral-sh/ty/blob/main/docs/README.md#python-environment
+    info: rule `unresolved-import` is enabled by default
+
+    Found 1 diagnostic
+
+    ----- stderr -----
+    WARN ty is pre-release software and not ready for production use. Expect to encounter bugs, missing features, and fatal errors.
+    ");
+
+    Ok(())
+}
+
 #[test]
 fn pyvenv_cfg_file_annotation_showing_where_python_version_set() -> anyhow::Result<()> {
     let case = CliTest::with_files([