test(custom-buffer-file): cover print spooler, diff, eww, email helpers

Sibling tests cover the move/rename/copy/clear family. The remaining helpers and dispatchers were uncovered:

- `cj/print--resolve-spooler`: explicit-string on PATH, explicit-string missing (error), `auto` prefers lpr, `auto` falls back to lp, `auto` with nothing on PATH (error), `auto` returns cached value, invalid spooler symbol (error).
- `cj/copy-buffer-name`: places the current buffer's name on the kill ring with a confirmation message.
- `cj/view-buffer-in-eww`: hands the file off to `eww-open-file` when visiting a file, errors otherwise.
- `cj/--email-handle-is-type-p`: prefix-matches content-types, handles nil handle without signaling.
- `cj/--diff-with-regular-diff`: writes a unified diff into the target buffer with header + diff output.
- `cj/diff-buffer-with-file`: errors when not visiting a file, messages "No differences" when diff -q returns 0.

16 new tests total. `call-process`, `executable-find`, `eww-open-file`, `mm-handle-type`, `diff-mode` are stubbed.

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