[201_90] fix inner product symbol display in tab cycle popup

[1sh-repalto]

Fixes #2926
Inner product symbol does not show on Tab cycling popup window

Summary

When cycling through math variants for the < key (which includes the inner product ⟨⟩ as the 7th variant), the popup window showed a broken/red symbol instead of the angle brackets.

Developer Document: devel/201_90.md

Issue Found

The math-bracket-open case in lambda-to-symbol was concatenating the left and right bracket symbols (e.g., "<langle><rangle>"). This produced a compound string that the widget-box rendering engine (using the roman font) could not render correctly, showing as a red broken symbol or --.

Changes

In TeXmacs/progs/math/math-edit.scm:

Modified lambda-to-symbol to return only the left bracket symbol for math-bracket-open:

 ((math-bracket-open)
  (and (>= (length (cdr body)) 2)
       (string? (cadr body))
       (string? (caddr body))
       (let ((lb (cadr body))
             (rb (caddr body)))
         `(symbol-completion
-          ,(string-append lb rb)))))
+          ,lb))))

How to test

1. Open Mogan Editor
2. Enter math mode by pressing $
3. Type <
4. Press Tab repeatedly (7 times) to reach the inner product ⟨⟩ variant
5. Verify that the popup window shows the left angle bracket ⟨ clearly (instead of a broken red symbol or --)
6. Press Tab once more to return to < and verify the cycle continues correctly