Report huge durations as invalid instead of raising decimal.Overflow

[cognis-digital]

Fixes #1511.

The duration format checker delegates to isoduration.parse_duration, which parses each numeric component with Decimal(...). When a component's magnitude exceeds the decimal context's Emax (e.g. P1E1000000D, or a digit run longer than Emax), Decimal raises decimal.Overflow. Because the checker was registered only with raises=isoduration.DurationParsingException, and decimal.Overflow is an ArithmeticError rather than a DurationParsingException, the exception propagated uncaught out of format validation instead of the instance being reported invalid.

Such strings are not valid RFC 3339 Appendix A durations (the grammar is 1*DIGIT, no exponent), so they should be reported invalid. This broadens the checker's caught exceptions to include decimal.Overflow.

Added regression tests for both the large-exponent and long-digit-run cases, guarded with skipUnless so they skip when the optional isoduration dependency isn't installed.

[read-the-docs-community]

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