test(gptel-tools): cover the helpers across the five remaining tools

The gptel-tools files had zero direct coverage outside of
`update_text_file`, which landed with its rewrite earlier this
session.  This commit adds 52 tests across the five other tools.

For three of the tools the helpers were already top-level defuns
(`read_text_file`, `list_directory_files`, `move_to_trash`).  The
other two had their main bodies inlined into the `gptel-make-tool`
lambda -- I extracted them so the work is testable without mocking
gptel itself:

  read_buffer.el     -> `cj/read-buffer--get-content`
  write_text_file.el -> `cj/write-text-file--run` plus
                        `--validate-path`, `--backup-name`,
                        `--ensure-parent`

Test files, by tool:

- read_buffer.el          (5 tests):  normal, empty, buffer-object,
  text-property-stripping, missing buffer.
- write_text_file.el     (10 tests):  validate-path, backup-name
  shape, ensure-parent (creates missing / rejects unwritable), run
  with normal / overwrite / existing-no-overwrite / empty content /
  outside-home.
- read_text_file.el      (12 tests):  validate-file-path (normal +
  three error shapes), metadata plist shape, size limits (no-op /
  hard cap / warning bypass with no-confirm), binary detection
  (text vs null-byte), special-type EPUB and generic-binary paths.
- list_directory_files.el (15 tests):  mode-to-permissions (file /
  dir / executable), get-file-info (file / directory), extension
  filter (keep / drop / always-dir / nil-extension), format-file-
  entry, list-directory flat / recursive / error, format-output
  with and without files.
- move_to_trash.el       (10 tests):  unique-name (no conflict /
  conflict with timestamp / no-extension), validate-path (HOME / /tmp
  / outside / critical-dir / missing), perform on file and
  directory.

Each test file uses the same load-path / gptel-stub idiom
(`eval-and-compile` block, gptel stub when the real package isn't
available) so the byte-compile hook is happy.

1 files changed, 33 insertions, 12 deletions

diff --git a/todo.org b/todo.org
index 8313d8bc..c0a9fa0e 100644
--- a/todo.org
+++ b/todo.org
@@ -2637,18 +2637,39 @@ the install-once guard for the post-response hook, and the
 save/delete interactive entry points exercised via =cl-letf= stubs.
 Per-test temp directories; no writes outside them.
 
-*** TODO [#B] Add ERT coverage for gptel-tools .el files :tests:
-
-The six tool files have zero direct coverage. Focus on the pure helpers (the interactive =gptel-make-tool= entry points get smoke coverage only):
-
-- =read_text_file.el= — =cj/validate-file-path=, =cj/get-file-metadata=, =cj/check-file-size-limits=, =cj/detect-binary-file=, =cj/handle-special-file-types=.
-- =update_text_file.el= — =cj/build-sed-command= (each operation × Normal / Boundary / Error), backup-naming behavior.
-- =write_text_file.el= — overwrite-vs-error path, backup naming, parent-directory creation.
-- =list_directory_files.el= — =--mode-to-permissions=, =--get-file-info=, =--filter-by-extension=, =--format-file-entry=, recursive vs flat listing on a temp dir.
-- =move_to_trash.el= — =gptel--move-to-trash-validate-path= (home/tmp allowed, anywhere else rejected), =gptel--move-to-trash-generate-unique-name= name-conflict suffixing.
-- =read_buffer.el= — small smoke test that =read_buffer= returns the body of an existing buffer and errors on a nonexistent name.
-
-Skip mocking =gptel-make-tool= itself; cover the helpers it wraps.
+*** 2026-05-16 Sat @ 01:39:11 -0500 Added ERT coverage for the gptel-tools .el files
+
+Five new test files cover the five remaining gptel tools beyond
+=update_text_file= (which was tested with its rewrite):
+
+- =tests/test-gptel-tools-read-buffer.el= -- 5 tests for the new
+  =cj/read-buffer--get-content= helper extracted from the
+  =gptel-make-tool= lambda.
+- =tests/test-gptel-tools-write-text-file.el= -- 10 tests for the
+  helpers extracted from =write_text_file.el= (validate-path,
+  backup-name, ensure-parent, run with normal/overwrite/error
+  paths).
+- =tests/test-gptel-tools-read-text-file.el= -- 12 tests for the
+  pre-existing helpers: =cj/validate-file-path=,
+  =cj/get-file-metadata=, =cj/check-file-size-limits=,
+  =cj/detect-binary-file=, =cj/handle-special-file-types=.
+- =tests/test-gptel-tools-list-directory-files.el= -- 15 tests for
+  the =list-directory-files--*= helpers (mode-to-permissions for
+  files/dirs/executables, get-file-info, extension filter, formatter,
+  recursive vs flat listing, error path).
+- =tests/test-gptel-tools-move-to-trash.el= -- 10 tests for the
+  =gptel--move-to-trash-*= helpers (unique-name generation with and
+  without extension, path validation gating HOME and /tmp, critical
+  directory rejection, perform on files and directories).
+
+Two small refactors landed first to make the tooling testable:
+=read_buffer.el= and =write_text_file.el= had their main bodies
+inlined into the =gptel-make-tool= lambdas; I extracted them into
+=cj/read-buffer--get-content= and =cj/write-text-file--run= (plus
+=--validate-path=, =--backup-name=, =--ensure-parent=) following the
+Internal/Wrapper split documented in =elisp-testing.md=.
+
+52 new tests, all green.
 
 *** TODO [#C] Research and shortlist additional gptel tools :feature:research: