Fix lookup_default returning UNSET sentinel instead of None

[veeceey]

Summary

Fixes #3145.

Since Click 8.3.0, Context.lookup_default returns the internal Sentinel.UNSET object instead of None when a parameter has no entry in the default map. This is a regression from the pre-8.3.0 behavior, where None was returned. The leak of the internal sentinel breaks:

• Subclasses of Context that override lookup_default and check if default is not None
• Any external code that calls lookup_default directly and expects None for missing defaults

Changes

• Extract _lookup_default (private): contains the original logic and returns UNSET for internal callers (Parameter.get_default and Parameter.consume_value) that need to distinguish "not set" from an explicit None value.
• Wrap lookup_default (public): delegates to _lookup_default and converts UNSET to None before returning, restoring the documented Any | None return type contract.
• Add regression test covering all cases: no default_map, missing key, present key, callable default, and a Context subclass matching the pattern from the issue report.

All 1320 existing tests continue to pass.

Test plan

• New test test_lookup_default_returns_none_not_sentinel passes
• Full test suite passes (1320 passed, 22 skipped, 1 xfailed)
• Verify the reproducer script from the issue no longer triggers AssertionError

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