Report overflowing durations as invalid instead of crashing

[aryansk]

What

Fixes #1511 — the duration format checker no longer crashes with an uncaught decimal.Overflow on strings whose amounts exceed Decimal's context limits; they are reported as invalid durations like any other malformed instance.

Why

isoduration.parse_duration builds each component amount with Decimal(...). For an exponent past the context's Emax — e.g. P1E1000000D — or a plain digit run longer than 999999, Decimal raises decimal.Overflow, which is not a subclass of the checker's declared isoduration.DurationParsingException. The exception escapes FormatChecker.check uncaught and propagates out of iter_errors/validate:

>>> v = Draft202012Validator({"format": "duration"}, format_checker=FormatChecker())
>>> list(v.iter_errors("P1E1000000D"))
# decimal.Overflow: [<class 'decimal.Overflow'>]  -- uncaught

The boundary is exact: P1E999999D is accepted, P1E1000000D overflows. The same bug is reachable without an exponent via a long digit run ("P" + "9" * 1000000 + "D").

How

decimal.Overflow is an ArithmeticError, so the checker's raises tuple is broadened to (isoduration.DurationParsingException, ArithmeticError) — the same idiom sibling checkers already use (e.g. raises=(idna.IDNAError, UnicodeError), and the pattern proposed in #1526). FormatChecker.check then wraps it into a FormatError and the instance is reported as invalid.

Tests

Added TestFormatChecker.test_it_rejects_durations_that_overflow_decimal (skipped when the optional isoduration dependency is not installed):

• P1E1000000D and the 1M-digit-run variant both raise FormatError (both crash with decimal.Overflow before the fix).
• P1Y2M3DT4H5M6S is still accepted.

Full test suite passes (497 tests). Added a CHANGELOG entry.

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