[runtime][python] Add Python durable reconciler support

[joeyutong]

Linked issue: #612

Purpose of change

This PR adds Python-side durable reconcile support based on a reconciler callable.

It builds on the Java durable reconciler runtime introduced in #600 and adds the Python API and runtime wiring needed to use the same recovery model from Python actions.

This PR includes:

• a new optional reconciler keyword-only argument on Python RunnerContext.durable_execute(...) and durable_execute_async(...)
• Python runtime support in FlinkRunnerContext for replaying previously completed durable results and recovering successful outcomes through a reconciler callable
• success-only reconciler semantics that match the merged Java contract:
  • returning a result persists and replays the recovered successful outcome
  • throwing an exception propagates that exception to the caller and does not persist a recovered terminal outcome
• local runner support that accepts the new reconciler argument and degrades to the existing non-durable behavior
• import-time decoupling from AgentPlan in Python runner modules, so lightweight runner tests do not eagerly pull in the heavier plan/action/PyFlink dependency chain
• a small Java bridge addition in RunnerContextImpl so the Python runtime can read the current durable call result fields during recovery

The detailed design discussion is available here:

• Discussion: [Feature] Durable Execution Reconcile #598

Tests

This change is covered by:

• test_durable_execution.py: verifies durable execution helpers and reconciler callable validation
• test_flink_runner_context_reconcilable.py: verifies recovered success, terminal replay, and exception propagation in the Flink runner context
• test_local_runner_reconcilable.py: verifies that local runner accepts the new reconciler argument and degrades to the existing non-durable behavior
• Java runtime compile: verifies the new bridge method used by the Python runtime compiles against the runtime module

API

Yes.

This PR introduces Python public API changes:

• RunnerContext.durable_execute(...) adds an optional keyword-only reconciler: Callable[[], Any] | None = None
• RunnerContext.durable_execute_async(...) adds the same optional reconciler keyword-only argument

Documentation

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