fix: lookup_default returns None instead of UNSET sentinel

[atulk-code]

Context.lookup_default() is a public API method that users can override or call directly. In Click 8.3.0, commit 1c20dc6 deferred UNSET normalization which caused lookup_default to return the internal UNSET sentinel instead of None when a parameter was not in the default_map.

This broke the public API contract - users expect None, not an internal sentinel object.

This fix:

• Adds internal _lookup_default() method that returns UNSET for internal use
• Modifies public lookup_default() to normalize UNSET to None
• Updates internal callers to use _lookup_default()
• Adds regression tests to verify the fix

Fixes #3145

[kdeldycke]

This looks a bit AI-generated. Don't know what pallets policy is on that front. In the mean time @atulk-code, can you confirm or deny it?

As for the solution, it is not bad. But the problem is adding a private _lookup_default() method, then referencing it in the code defeat the purpose of having a public lookup_default() in Click API. What I mean is that we expect Click itself to invoke the public lookup_default() everywhere in its code, so the user has the opportunity to alter its behavior. This is exactly what the user was complaining about in #3145.

You should add a unittest that is reproducing #3145 exactly, with its custom CustomClickContext. lookup_default method to understand the limitations of your solution.

[davidism]

This is like the 6th person to AI generate this PR. I'm not interested in that.

[kdeldycke]

I'm back from other rabbit holes so I'll try to get back to work on Click. I guess I can fix that for good based on my feedback.