Raise DurationParsingException on decimal overflow instead of leaking decimal.Overflow

[cognis-digital]

parse_duration() promises to raise DurationParsingException for any malformed input,
but a value whose magnitude overflows the decimal context escapes that contract with a
raw decimal.Overflow:

>>> from isoduration import parse_duration
>>> parse_duration("P1E1000000D")
decimal.Overflow: [<class 'decimal.Overflow'>]

is_number() accepts scientific-notation e/E, so 1E1000000 is accepted as a numeric
token. Decimal("1E1000000") constructs without issue, but the subsequent
sign * Decimal(...) runs under the default decimal context (Emax 999999) and signals
Overflow. Both arithmetic sites in parser/parsing.py only caught InvalidOperation, so
the overflow bypassed the library's exception hierarchy — a problem for callers such as
jsonschema's duration format check that catch only DurationParsingException.

This broadens the handler at both sites from decimal.InvalidOperation to its base class
decimal.DecimalException. Since InvalidOperation is a subclass of DecimalException,
all previously handled inputs behave identically; overflow (and any other decimal signal)
is now reported as UnparseableValue, consistent with other unparseable numeric values.

Adds regression tests for both the date (P1E1000000D) and time (PT1E1000000S) paths.

Full test suite passes (378 passed).