Extract provider and token modules into codex-auth

Move the foundational provider and token modules into codex-auth while keeping codex-core as the facade. Also move the corresponding unit tests and record the 3-PR migration checkpoints.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/codex-auth/src/error.rs

@@ -0,0 +1,15 @@
+#[derive(Debug)]
+pub struct EnvVarError {
+    pub var: String,
+    pub instructions: Option<String>,
+}
+
+impl std::fmt::Display for EnvVarError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "Missing environment variable: `{}`.", self.var)?;
+        if let Some(instructions) = &self.instructions {
+            write!(f, " {instructions}")?;
+        }
+        Ok(())
+    }
+}