Fix two follow-up issues: reject nonexistent roots, single-source extensions

- pipeline: fail index create/update with a clear error when the project root
  is not a directory instead of building a misleading zero-file index
- config: single LANGUAGE_BY_EXT table as source of truth; language_for and
  resolver code_extensions both derive from it (no more duplicated ext list)
- imports: resolver probes code_extensions() instead of a local RESOLVE_EXTS
- tests: regression for nonexistent-root rejection

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/cli.rs

@@ -626,3 +626,21 @@ fn mcp_reports_invalid_params_and_parse_errors() {
         .expect("parse error response");
     assert_eq!(parse["error"]["message"], "parse error");
 }
+
+#[test]
+fn index_create_rejects_nonexistent_root() {
+    let out = Command::new(bin())
+        .args(["index", "create", "/definitely/not/a/real/rustytokens/path"])
+        .env("XDG_CACHE_HOME", cache_dir())
+        .output()
+        .expect("run rustytokens");
+    assert!(
+        !out.status.success(),
+        "index create must fail on a nonexistent root"
+    );
+    assert!(
+        String::from_utf8_lossy(&out.stderr).contains("not a directory"),
+        "stderr explains the failure: {}",
+        String::from_utf8_lossy(&out.stderr)
+    );
+}