feat(gptel-tools): harden path validation with file-truename realpath

Resolves PATH through file-truename before applying home-directory and
read/write checks across the path-handling tools (git_status, git_log,
git_diff, move_to_trash, read_text_file, update_text_file,
write_text_file, list_directory_files, read_buffer, web_fetch).
Without the resolve step, a symlink under HOME pointing outside HOME
would pass the prefix check but the tool would act on the real target
-- a symlink-escape.

move_to_trash also tightens the trash-bin construction (treats empty
file extensions correctly) and switches the "critical directories"
list to truename-resolved canonical forms so a symlinked ~/.config
can't be trashed via an aliased path.

update_text_file fixes an off-by-one in the line-count derivation
when the source content is empty.

Each source change pairs with tests in tests/test-gptel-tools-*.el
and tests/test-update-text-file.el covering the realpath escape
paths, the empty-extension trash case, and the empty-content line-
count edge.  Combined coverage is now 100% across all ten gptel-tools
source files: 516 / 516 executable lines, 217 tests.

1 files changed, 26 insertions, 0 deletions

diff --git a/tests/test-gptel-tools-git-diff.el b/tests/test-gptel-tools-git-diff.el
index 59666a32..114fec29 100644
--- a/tests/test-gptel-tools-git-diff.el
+++ b/tests/test-gptel-tools-git-diff.el
@@ -102,6 +102,26 @@
         (should-error (cj/gptel-git-diff--validate-path dir))
       (when (file-exists-p dir) (delete-directory dir t)))))
 
+(ert-deftest test-gptel-tools-git-diff-validate-path-error-not-a-directory ()
+  "Error: file paths are rejected."
+  (let ((file (make-temp-file
+               (expand-file-name ".test-gptel-tools-git-diff-file-" "~"))))
+    (unwind-protect
+        (should-error (cj/gptel-git-diff--validate-path file))
+      (when (file-exists-p file) (delete-file file)))))
+
+(ert-deftest test-gptel-tools-git-diff-validate-path-error-symlink-outside-home ()
+  "Error: symlinked directories resolving outside HOME are rejected."
+  (let ((link (expand-file-name
+               (format ".test-gptel-tools-git-diff-link-%s"
+                       (format-time-string "%s%N"))
+               "~")))
+    (unwind-protect
+        (progn
+          (make-symbolic-link "/tmp" link t)
+          (should-error (cj/gptel-git-diff--validate-path link)))
+      (when (file-symlink-p link) (delete-file link)))))
+
 ;; ---------- run
 
 (ert-deftest test-gptel-tools-git-diff-run-no-changes ()
@@ -133,5 +153,11 @@
        (should (string-match-p "f.txt" out))
        (should-not (string-match-p "g.txt" out))))))
 
+(ert-deftest test-gptel-tools-git-diff-run-error-on-bad-ref ()
+  "Error: git diff exits other than 0/1 are surfaced."
+  (test-gptel-tools-git-diff--with-repo
+   (lambda (dir)
+     (should-error (cj/gptel-git-diff--run dir "does-not-exist")))))
+
 (provide 'test-gptel-tools-git-diff)
 ;;; test-gptel-tools-git-diff.el ends here