refactor(config): remove unused config types

[ayushiahjolia]

Issue #, if available:
#532

Description of changes:
Removes dead public surface from the SDK: config types that accept input but are never read, and error-type enums that were never wired into any path. All are non-functional today, so removal only changes the importable
public surface (none are exported from __init__.py). Targets v2.

Unused config types

• Remove ItemBatcher, ItemsPerBatchUnit, BatchedInput and MapConfig.item_batcher; simplify the map callable to Callable[[DurableContext, U, int, Sequence[U]], T], matching JS MapFunc.
• Remove TerminationMode (no SDK has an equivalent).
• Remove StepFuture (mirrors JS DurablePromise; never adopted by the synchronous Python model).
• Remove ChildConfig.item_serdes and its stale backward-compat docstring. MapConfig.item_serdes / ParallelConfig.item_serdes are unaffected (still read by the concurrency executor).
• Remove WaitStrategyConfig.timeout and its timeout_seconds property. create_wait_strategy never reads it; the strategy is client-side with no backend enforcement, and neither the Java nor JS SDK has a working equivalent.

Dead error-type enums

• Remove ChainedInvokeFailedToStartType, ChainedInvokeTimeoutType and ChainedInvokeStopType from lambda_service.py. The invoke executor already classifies terminal failures by checkpoint status and carries the error type through from the backend ErrorObject.

Test cleanup

• Consolidate the suspend_with_optional_resume_delay helper tests: drop the four duplicates from invoke_test.py and cover none/negative/zero/positive in one parametrized test in suspend_test.py.

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[yaythomas]

nit: config.py:423 (InvokeConfig) — while we're removing dead config surface, the commented-out # retry_strategy: ... line above the fields could go too (pre-existing, optional). (Posting here because the line isn't in the diff.)

[yaythomas]

Verified every removed symbol has zero residual references across all five packages, the examples, and the docs repo, and that MapConfig.item_serdes/ParallelConfig.item_serdes remain live in the concurrency executor. 1421 tests, typecheck, and fmt all clean locally. The ChainedInvoke*Type removal beyond the #532 list is also correct — nothing consumes them and the invoke executor classifies terminal failures from the checkpoint status, with the error type carried through from the backend error object.

Nice catch tightening the zero-delay suspend assertion to TimedSuspendExecution — the deleted duplicate in invoke_test.py asserted the broader SuspendExecution, which only passed via subclassing. The parametrized version now pins the actual behavior (from_delay(msg, 0)).

Thank you very much :-)

Pretty much ready to approve, just those 2 nits if possible :-)