docs(serialization): document first-run round trip and retryable serdes error

[ayushiahjolia]

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

Description of changes:
Document two 2.x serialization behaviors:

• First-run round trip: step, child contexts, map, parallel, and wait_for_condition now serialize then deserialize the result on the first run, so first-run output matches replay. invoke and wait are unaffected.
• A non-identity custom SerDes now changes the first-run value too.
• Python note: wait_for_condition round-trips initial_state.
• Serdes errors: a custom SerDes raises SerDesError for a permanent failure (no retry) or RetryableSerDesError for a transient one (fails the invocation so the backend retries).

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

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[yaythomas]

This language-neutral section is not accurate for every SDK.

In the current C# SDK, step, child-context, wait-for-condition, map, and parallel operations serialize their checkpoint but return the raw first-run value; they deserialize only on replay. C# also uses a single ILambdaSerializer, not a per-operation custom SerDes.

Java defaults to round-tripping but allows it to be disabled with DurableConfig.withDeserializeAfterSerialization(false).

Please move this behavior into language tabs: document TypeScript/Python round-tripping, Java’s configurable behavior, and C#’s raw-first-run/replay-deserialization behavior. This will also remove the Python-specific wait_for_condition spelling from shared prose.