fix: deflake hooks-ordering contract test

[beekld]

Summary

Fixes flaky contract tests hooks/evaluation/executes beforeEvaluation hooks in registration order and the corresponding afterEvaluation reverse-order test, which had been intermittently red on CI.

Root cause: ContractTestHook::PostCallback was firing the harness notifications as fire-and-forget async chains (resolve -> connect -> write). The SDK invokes hooks sequentially, but the three async POSTs then race on the wire, so the harness can receive them in any order.

Fix: send each callback synchronously via the boost::beast sync HTTP API. The sync path uses blocking syscalls rather than the reactor, so there is no deadlock on the single-thread io_context, and each POST completes before the next hook's call starts.

Local repro on macOS over loopback: 2/50 fails before, 0/100 after.

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Note

Low Risk
Test-only contract hook infrastructure; no production SDK behavior change beyond blocking briefly during test callbacks.

Overview
ContractTestHook::PostCallback no longer uses fire-and-forget async resolve/connect/write/read chains. Each harness notification is sent with synchronous Boost.Beast HTTP (resolve, connect, write, read, shutdown) so a POST finishes before the next hook stage runs.

That preserves callback arrival order when the SDK invokes hooks sequentially, which stabilizes hooks-ordering contract tests that assert registration order for beforeEvaluation / afterEvaluation.

The rest of the diff is import reordering and minor JSON serialization formatting; hook behavior is unchanged aside from callback delivery semantics.

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