[Testing] Add deterministic ALSP spam-decision boundary tests

[janezpodhostnik]

Follow-up to #8626/#8627. The widened statistical bounds in the sync spam load tests left
the low-probability groups unable to detect an implementation that never reports
misbehavior (their lower bound is 0). This PR replaces that statistical coverage with
exact unit tests.

Changes

• Extracted pure helpers in engine/common/synchronization/engine.go
  (behavior-preserving — same expressions, same float32 semantics):
  shouldReportProbabilistically, batchRequestMisbehaviorProbability,
  rangeRequestMisbehaviorProbability. The probabilistic decision was previously inlined
  3× and untestable deterministically (hardwired crypto RNG).
• New deterministic table tests (engine_alsp_probability_test.go): never report at
  p=0, always at p=1, draw n=0 reports for any p>0 (the case the load tests could not
  discriminate), exact boundaries at p=0.5 and p=0.1, and the documented (n+1)/64
  scaling example values for range/batch requests.
• Slimmed the statistical load tests to wiring smoke tests: kept only groups with
  discriminating power (p=0.0, p=0.5, p≈0.156, and the unconditional always-report
  groups); dropped the eleven low-probability groups whose bounds could not fail for a
  broken implementation. Cuts ~11,000 processed messages from the suite.

Package passes go test -race (repeated runs); lint clean.

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