test(file-tool): cover parse_pages_arg edge cases

`parse_pages_arg` validates the user-supplied `pages` argument that
`ReadFileTool` forwards to `pdftotext -f START -l END`. The function
has zero tests today even though it's the only gatekeeper between
user input and a pdftotext spawn — silent acceptance of a malformed
range yields a confusing empty extraction with no actionable error
message.

Adds five tests:

* `parse_pages_arg_accepts_single_page` — `"3"` and `" 7 "` both
  return `Some((n, n))`.
* `parse_pages_arg_accepts_range` — `"1-5"`, `"10-20"`, and
  whitespace-tolerant `" 1 - 5 "` all parse correctly.
* `parse_pages_arg_rejects_invalid_ranges` — `5-1` (end < start),
  `0` and `0-3` (one-indexed contract), empty / whitespace-only
  inputs, `abc` (non-numeric), and `3.5` (floats) all return `None`.
* `parse_pages_arg_rejects_half_open_ranges` — `1-`, `-5`, and `-`
  reject rather than silently extending to `u32::MAX` or `0`.
* `parse_pages_arg_rejects_negative_numbers` — `-3-5` doesn't wrap
  into a giant positive number via u32 parsing.

Zero behaviour change; locks the contract so a future innocuous edit
can't silently shift validation.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/tui/src/tools/file.rs

@@ -462,6 +462,58 @@ mod tests {
         assert_eq!(result.content, "hello world");
     }
 
+    #[test]
+    fn parse_pages_arg_accepts_single_page() {
+        assert_eq!(parse_pages_arg("3"), Some((3, 3)));
+        assert_eq!(parse_pages_arg("  7  "), Some((7, 7)));
+    }
+
+    #[test]
+    fn parse_pages_arg_accepts_range() {
+        assert_eq!(parse_pages_arg("1-5"), Some((1, 5)));
+        assert_eq!(parse_pages_arg("10-20"), Some((10, 20)));
+        // Whitespace around either side of the dash is tolerated so
+        // hand-typed `pages: "1 - 5"` still works.
+        assert_eq!(parse_pages_arg(" 1 - 5 "), Some((1, 5)));
+    }
+
+    #[test]
+    fn parse_pages_arg_rejects_invalid_ranges() {
+        // Caller would otherwise feed `pdftotext -f 5 -l 1`, which
+        // prints nothing — fail loudly so the model can re-issue.
+        assert!(parse_pages_arg("5-1").is_none(), "end < start must reject");
+        // 0-indexed pages aren't a thing in pdftotext; reject so the
+        // caller doesn't get a confusing "no output" silent fail.
+        assert!(
+            parse_pages_arg("0").is_none(),
+            "zero single-page must reject"
+        );
+        assert!(parse_pages_arg("0-3").is_none(), "zero start must reject");
+        // Empty / whitespace-only / non-numeric inputs must reject.
+        assert!(parse_pages_arg("").is_none());
+        assert!(parse_pages_arg("   ").is_none());
+        assert!(parse_pages_arg("abc").is_none());
+        assert!(parse_pages_arg("3.5").is_none(), "floats must reject");
+    }
+
+    #[test]
+    fn parse_pages_arg_rejects_half_open_ranges() {
+        // Half-open ranges like `1-` or `-5` are almost certainly a
+        // typo for `1-N`/`N` rather than intentional input. Reject
+        // them rather than silently extending to u32::MAX or 0.
+        assert!(parse_pages_arg("1-").is_none());
+        assert!(parse_pages_arg("-5").is_none());
+        assert!(parse_pages_arg("-").is_none());
+    }
+
+    #[test]
+    fn parse_pages_arg_rejects_negative_numbers() {
+        // u32::parse on a negative literal returns Err, so the
+        // function reports `None` rather than wrapping into a giant
+        // positive number — defensive but worth pinning.
+        assert!(parse_pages_arg("-3-5").is_none());
+    }
+
     #[tokio::test]
     async fn test_read_file_not_found() {
         let tmp = tempdir().expect("tempdir");