warn when pager is missing instead of silently falling back

when a configured pager (via BAT_PAGER, PAGER, or --pager) is not found,
bat now shows a warning message before falling back to stdout. this helps
users understand why their pager isn't running and makes it obvious when
there's a typo or PATH issue.

fixes issue #2904

tests/integration_tests.rs

@@ -1425,6 +1425,21 @@ fn pager_failed_to_parse() {
         .stderr(predicate::str::contains("Could not parse pager command"));
 }
 
+#[test]
+#[serial]
+fn pager_missing_warning() {
+    bat()
+        .env("BAT_PAGER", "nonexistent-pager-xyz-missing")
+        .arg("--paging=always")
+        .arg("test.txt")
+        .assert()
+        .success()
+        .stderr(predicate::str::contains("[bat warning]"))
+        .stderr(predicate::str::contains("not found"))
+        .stderr(predicate::str::contains("nonexistent-pager-xyz-missing"))
+        .stdout(predicate::str::contains("hello world\n"));
+}
+
 #[test]
 #[serial]
 fn env_var_bat_paging() {