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.

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