Validate CODEX_HOME before resolving (#10249)

Summary
- require `CODEX_HOME` to point to an existing directory before
canonicalizing and surface clear errors otherwise
- share the same helper logic in both `core` and `rmcp-client` and add
unit tests that cover missing, non-directory, valid, and default paths

This addresses #9222

codex-rs/core/src/config/mod.rs

@@ -57,7 +57,6 @@ use codex_protocol::openai_models::ReasoningEffort;
 use codex_rmcp_client::OAuthCredentialsStoreMode;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use codex_utils_absolute_path::AbsolutePathBufGuard;
-use dirs::home_dir;
 use schemars::JsonSchema;
 use serde::Deserialize;
 use serde::Serialize;
@@ -1753,27 +1752,12 @@ fn toml_uses_deprecated_instructions_file(value: &TomlValue) -> bool {
 /// specified by the `CODEX_HOME` environment variable. If not set, defaults to
 /// `~/.codex`.
 ///
-/// - If `CODEX_HOME` is set, the value will be canonicalized and this
-///   function will Err if the path does not exist.
+/// - If `CODEX_HOME` is set, the value must exist and be a directory. The
+///   value will be canonicalized and this function will Err otherwise.
 /// - If `CODEX_HOME` is not set, this function does not verify that the
 ///   directory exists.
 pub fn find_codex_home() -> std::io::Result<PathBuf> {
-    // Honor the `CODEX_HOME` environment variable when it is set to allow users
-    // (and tests) to override the default location.
-    if let Ok(val) = std::env::var("CODEX_HOME")
-        && !val.is_empty()
-    {
-        return PathBuf::from(val).canonicalize();
-    }
-
-    let mut p = home_dir().ok_or_else(|| {
-        std::io::Error::new(
-            std::io::ErrorKind::NotFound,
-            "Could not find home directory",
-        )
-    })?;
-    p.push(".codex");
-    Ok(p)
+    codex_utils_home_dir::find_codex_home()
 }
 
 /// Returns the path to the folder where Codex logs are stored. Does not verify