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

Jforresterrequested to merge
T416161-allows-callbacks into main

[Merge after !674 (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 orchestrator now requires and emits the 2.1.1 exchange format.

Internal mechanism names (useReentrance_, evaluateReentrant_, reentrantCalled test variable, the Z563 error constant) are left unchanged: they describe the websocket-based callback mechanism rather than the wire-format field.

Test fixtures in herz/evaluator.json and transpilation/evaluatedTests.js update accordingly, and the mock evaluator's recoverRequestFromExchangeFormat import is bumped to 2.1.1 so it can parse the renamed field.

Bug: T416161

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

Edited by Jforrester

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