Common base class for CEL exceptions, without breaking the existing builtin mappings

首先检查 python/cel/exceptions.py、map_execution_error_to_python、cel.pyi 和错误处理操作指南。跟踪现有的执行错误和解析错误映射，然后验证新的层次结构是否保留内置异常处理、支持捕获 cel.CelError，并在存根和操作指南表格中记录相关内容。

Follow-up promised when closing #23. Execution errors are mapped to the idiomatic builtin (RuntimeError, TypeError, KeyError, IndexError, ZeroDivisionError, OverflowError) and parse errors to ValueError, which is right for each case but means "did the rule run and fail" needs a five-clause except:

except (RuntimeError, TypeError, KeyError, IndexError, ArithmeticError):

Proposal

Add a cel.CelError hierarchy whose members also inherit from the builtin they replace, so nothing existing breaks:

class CelError(Exception): ...
class CelParseError(CelError, ValueError): ...
class CelRuntimeError(CelError, RuntimeError): ...
class CelTypeError(CelError, TypeError): ...
class CelKeyError(CelError, KeyError): ...
class CelIndexError(CelError, IndexError): ...
class CelZeroDivisionError(CelError, ZeroDivisionError): ...
class CelOverflowError(CelError, OverflowError): ...

except TypeError keeps working; except cel.CelError catches everything CEL raised; except cel.CelParseError separates a bad rule from a failed check.

Implementation notes

PyO3's create_exception! only takes a single base, so define the classes in a small python/cel/exceptions.py and have map_execution_error_to_python look them up from the module once (a GILOnceCell<Py<PyType>> per class) and raise with PyErr::from_type. Add them to cel.pyi and to the error-handling how-to, whose current table becomes the mapping between the two hierarchies.