fix(docx_math): preserve multiple arguments in delimiter (<m:d>) and support sepChr (#2573)

[İlker Oğulcan SAYIN (iosayin)]

Closes #2573

Summary of Changes

• In oMath2Latex.do_d, collects all <m:e> base arguments in order using process_children_list instead of process_children_dict (which previously overwrote all earlier arguments with the last one).
• Adds sepChr to Pr.__val_tags and Pr.tag2meth to read the delimiter separator character (with empty string support if m:val is omitted).
• Joins the delimiter arguments using sepChr when specified, or defaults to | per the OMML / ECMA-376 specification.
• Adds comprehensive unit tests covering explicit separator (,), default separator (|), and multi-argument delimiters.