chore: improve key parsing logic and fix premature termination issues

[famouswizard]

fixed several parsing bugs in key handling logic:

• find_key_end_position: removed '/' from the set of terminating characters. This prevents derivation paths like .../44'/0'/0'/... from being truncated at the first /.
• parse_key_expressions: implemented proper parenthesis depth tracking for musig(...) expressions instead of stopping at the first ), allowing correct parsing of nested expressions.
• parse_key_expressions: added boundary checks (key_pos_end < len(descriptor)) before indexing descriptor[key_pos_end] to avoid IndexError at string ends.

these changes ensure the parser correctly handles complex key expressions and avoids crashes on edge cases.