fix: guard parseBusName against non-numeric bus subscripts

[openroad-ci]

Problem:
Reading a DEF with port names containing non-numeric bracket subscripts (e.g., kv_write[3][write_en]) crashes with a segfault. parseBusName uses rfind('[') to find the last left bracket, then calls std::stoi on the content. For kv_write[3][write_en], rfind finds the [ before write_en, and stoi("write_en]") throws an unhandled std::invalid_argument which terminates the process.

This occurs when externally-produced DEF files use multi-level bracket names where the innermost subscript is a field name rather than a numeric bus index.

Fix:
Add an isdigit check on the first character after [ before calling stoi in both parseBusName overloads. Non-numeric subscripts like [write_en] are treated as non-bus names (is_bus = false), while ports with numeric trailing subscripts like kv_write[3][ write_entry][4] continue to parse and group correctly as bus kv_write[3][write_entry] with index 4.