[Eglot] Stricter "expand common" behavior

[Eglot] Stricter "expand common" behavior

* lisp/progmodes/eglot.el (eglot--dumb-tryc): Check that the
expanded string matches every completion strictly (bug#72705).
And in the fallback case, check whether the table matches the
original prefix at all.  Return nil otherwise.

* test/lisp/progmodes/eglot-tests.el
(eglot-test-stop-completion-on-nonprefix)
(eglot-test-try-completion-nomatch): Corresponding tests.

* etc/EGLOT-NEWS: New entry.

(cherry picked from commit 713069dd7a87cff8388c25f1bc2c2c1b5217b1ca)