docs(errors): rebuild Python exception hierarchy for 2.x

[ayushiahjolia]

Issue #, if available: aws/aws-durable-execution-sdk-python#663

Description of changes:
The Python error hierarchy on the error-handling page described the 1.x tree.
Update it to 2.x: drop the removed UserlandError and CallableRuntimeError, add the DurableOperationError base (StepError,
InvokeError, ChildContextError, WaitForConditionError, BatchCompletionError, and the graded CallbackError subtypes), reparent SerDesError under DurableExecutionsError, and add RetryableSerDesError. Updates the mermaid diagram, prose, and the exception-hierarchy.py example.

Checks: mdformat --check, codespell, check_example_refs.py, and zensical build --clean all pass.

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[yaythomas]

the Python tab under Serialization errors still says that serialization or deserialization failures always raise SerDesError and do not retry. Please revise that section to distinguish permanent failures (SerDesError, no retry) from transient failures (RetryableSerDesError, fail the invocation so the backend retries). Otherwise this page contradicts itself, as well as the new serialization guidance in #275.