feat: deprecate the unused CacheError

[allen0099]

Closes #163. Part of #113. Removal is tracked in #130 (0.4.0).

fastapi_cachex.exceptions.CacheError is never raised anywhere in the package.

• The class is renamed to the private _CacheError (with __name__/__qualname__ kept as CacheError) and served through a module-level __getattr__ (PEP 562). Both exceptions.CacheError and from fastapi_cachex.exceptions import CacheError emit a DeprecationWarning that points at the caller's line (stacklevel=2). It is still a CacheXError subclass, so except CacheError keeps working.
• A TYPE_CHECKING alias keeps CacheError visible to type checkers.
• import fastapi_cachex does not trigger the warning (checked with -W error::DeprecationWarning).
• The API reference no longer lists the class, so docs/api/types.md gets a short deprecation note.
• CHANGELOG: new ### Deprecated entry. It also removes the blank line between the two existing Deprecated entries, to match the rest of the file.

Tests

New tests/test_exceptions.py: the warning on attribute access (including the caller's filename), on from ... import, and AttributeError for unknown names. Mutation checks: removing the warning fails only the two warning tests; stacklevel=1 fails only the filename test. Full suite, ruff, mypy --strict, the changelog format test and both --strict --clean docs builds pass.