[docs] Document durable reconciler usage

[joeyutong]

Linked issue: #612

Purpose of change

This PR includes:

• documentation for the durable reconciler contract in both Python and Java
• clarification that a reconciler can recover either a successful result or a failed outcome, and that the runtime persists and replays either terminal outcome
• usage examples for reconciler / reconciler() in the workflow agent guide, including reconciler branches that return recovered success or raise recovered failure
• clarification that reconciler-backed durable execution does not change the existing async support matrix
• documentation that reconciler-based recovery relies on the persisted action state used for exactly-once action consistency
• an additional clarification that durable-call replay and action-level replay are different: code outside durable execution is re-executed only when recovery re-enters an action that has not been recorded as completed

Tests

Not run. This is a documentation-only change.

API

No.

Documentation

• doc-needed
• doc-not-needed
• doc-included

[xintongsong]

It feels a bit obscure. Might be better to start with the two questions that users care most: when to use this feature (call started but not completed/persisted, framework cannot decide should replay or not), and what it can do (support custom reconciling logics).

[xintongsong]

LGTM