docs(map,parallel): document completion predicate and fix 2.x semantics

[ayushiahjolia]

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

Description of changes:
Bring the Python map and parallel completion docs to 2.x:

• Add should_complete to the Python CompletionConfig, with a Custom-completion-predicate walkthrough (complete_batch / continue_batch, CompletionStatus, CompletionOutcome, BatchCompletionError) and new custom-completion.py examples.
• Fix throw_if_error: it raises ChildContextError / SerDesError / BatchCompletionError, not the removed CallableRuntimeError.
• Correct the BatchResult.all semantics: started-but-incomplete items appear as STARTED; never-started items are omitted, so total_count excludes them.
• Remove the false "all_completed() is deprecated" note; document the fail-fast default and all_completed() as the run-everything option.

Checks: mdformat, codespell, check_example_refs, zensical build all pass.

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