Fix lookup_default return None

[StevOti]

Fixes #3145

Problem

In Click >= 8.3, Context.lookup_default() returns the internal
Sentinel.UNSET object when a key is missing. Historically (<= 8.2.1),
the public API returned None.

Reproduction

• main: ctx.lookup_default("missing_key") -> Sentinel.UNSET
• Click 8.2.1: ctx.lookup_default("missing_key") -> None

Root Cause

Regression introduced by 1c20dc6 (“Fix default handling to defer UNSET normalization ”)
Let the internal sentinel leak through the public API.

Solution

• Add internal _lookup_default() that returns UNSET for internal use
• Normalize public lookup_default() to return None
• Update internal call sites to use _lookup_default()
• Add regression tests covering missing keys, callable defaults, and internal behavior

Safety

• Internal sentinel semantics preserved
• Regression tests added
• Full test suite run locally (see note below)

[StevOti]

I verified the regression and historical behaviour:

• main: lookup_default("missing_key") -> Sentinel.UNSET
• Click 8.2.1: lookup_default("missing_key") -> None

Full test suite on Windows/Python 3.14.2: 1 failure in tests/test_imports.py::test_light_imports
because ctypes._layout is imported (not in ALLOWED_IMPORTS). This seems unrelated
to the lookup_default change; happy to rerun in a different env if preferred.