fix(diagnostics): eglot with flycheck vs flymake

[DavidVujic]

The editor_diagnostics failed with Symbol's function definition is void: flymake-diagnostic-code and it happens when running eglot + flycheck (no lsp-mode).

Suggested solution:
checking if flycheck is the active tool used before running the flymake diagnostics.

Current workaround in my Emacs setup:

(with-eval-after-load 'eca-editor
  (defun eca-editor--flymake-diagnostics (_uri _workspace)
    "Neutralized: flycheck-eglot already bridges eglot diagnostics into flycheck."
    nil))

Note: the changes in this PR were created by Eca with the Opus 4.7 model - with me steering. I am not confident in my Elisp skills as I am in Python, though.

[ericdallo]

Looks good, thank you!