Evaluator: Switch to exchange format 2.1.1, renaming 'reentrant' as 'allowsCallbacks'

Jforresterrequested to merge
T416161-allows-callbacks into main

[Merge after !540 (merged).]

The wire-format field that signals whether the caller will accept callbacks from inside an evaluation is renamed from 'reentrant' to 'allowsCallbacks' for clarity. The evaluator now expects requests in the 2.1.1 exchange format.

Internal executor-callback mechanism names (START_REENTRANT_CALL_REGEX_, reentrantCallQueue_, the Python wikifunctions.py log markers, the non-reentrant-evaluator test fixture, the Z563 error constant) are left unchanged: they describe the executor-to-orchestrator callback protocol rather than the wire-format field.

test_data/exchange_format_2_1_0/ is renamed to exchange_format_2_1_1/ with the field rename and schemaVersion bump applied. executor test fixtures and integrationTest.js (which round-trips through the recover function) are updated to match.

Bug: T416161

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com

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