fix: improve enum misuse error

[Sohel2309]

Bug

Using a non-Enum value with an enum-typed attrs field can result in an
unhelpful AttributeError: 'str' object has no attribute 'value'.

Root cause

enum_unstructure_factory() generated unstructure hooks that accessed
.value without first checking that the value was an instance of the
expected Enum type.

Fix

Add an isinstance() check for the expected Enum type before accessing
.value. Invalid values now raise a clear TypeError describing the
expected Enum type and the received value.

The fix covers both the regular Enum and typed-enum paths.

Tests

Added regression tests covering:

• regular Enum misuse
• typed Enum misuse
• the attrs end-to-end case from issue Enum misuse causes unhelpful error #601

The regression tests were verified to fail before the fix and pass after
the fix.

Changelog

Added the fix to HISTORY.md.

[Tinche]

The problem is this inserts an isinstance check into every unstructure call, affecting performance. cattrs expects the type hints to be correct, so I'm not inclined to trade performance for better error messages in this particular case.