test(ptx): Add regression tests for TeX and chunking bugs

Adds two fixture-based tests to verify GNU-compatible behavior:

- `test_tex_format_no_truncation_markers`: Ensures TeX format does not
  display truncation markers.
- `gnu_ext_disabled_chunk_no_over_reading`: Verifies that text chunks
  do not over-read at boundaries in traditional mode.

tests/by-util/test_ptx.rs

@@ -11,6 +11,25 @@ fn test_invalid_arg() {
     new_ucmd!().arg("--definitely-invalid").fails_with_code(1);
 }
 
+#[test]
+fn test_tex_format_no_truncation_markers() {
+    let input = "Hello world Rust is a fun language";
+    new_ucmd!()
+        .args(&["-G", "-w", "30", "--format=tex"])
+        .pipe_in(input)
+        .succeeds()
+        .stdout_only_fixture("test_tex_format_no_truncation_markers.expected");
+}
+#[test]
+fn gnu_ext_disabled_chunk_no_over_reading() {
+    let input = "Hello World Rust is a fun language";
+    new_ucmd!()
+        .args(&["-G", "-w", "30"])
+        .pipe_in(input)
+        .succeeds()
+        .stdout_only_fixture("gnu_ext_disabled_chunk_no_over_reading.expected");
+}
+
 #[test]
 fn test_truncation_no_extra_space_in_after() {
     new_ucmd!()

tests/fixtures/ptx/gnu_ext_disabled_chunk_no_over_reading.expected

@@ -0,0 +1,7 @@
+.xx "Rust is/" "" "Hello World" ""
+.xx "" "Hello World" "Rust is a/" ""
+.xx "is/" "Hello" "World Rust" ""
+.xx "" "/Rust is" "a fun/" ""
+.xx "" "/Rust is a" "fun language" ""
+.xx "" "/World Rust" "is a fun/" ""
+.xx "" "/is a fun" "language" ""

tests/fixtures/ptx/test_tex_format_no_truncation_markers.expected

@@ -0,0 +1,7 @@
+\xx {Rust is}{}{Hello}{ world}{}
+\xx {}{Hello world}{Rust}{ is a}{}
+\xx {}{Rust is}{a}{ fun}{}
+\xx {}{Rust is a}{fun}{ language}{}
+\xx {}{world Rust}{is}{ a fun}{}
+\xx {}{is a fun}{language}{}{}
+\xx {is}{Hello}{world}{ Rust}{}