[TST] Replace timing-based concurrency assertions with deterministic synchronization

[tazarov]

Summary

Some concurrency tests in /Users/tazarov/GolandProjects/onnx-purego/ort/session_test.go still rely on fixed timeout windows (currently 500ms) to assert blocking behavior.

These assertions can become flaky under scheduler/CI load even when behavior is correct.

Context

Relevant tests include:

• TestAdvancedSessionRunAndDestroyConcurrent
• TestAdvancedSessionDestroyDoesNotBlockUnrelatedRun
• TestTensorDestroyWaitsForInFlightRun

Goal

Use deterministic channel/handshake synchronization (or explicit state signaling) instead of timeout-based inference whenever possible.

Proposed changes

1. Introduce explicit rendezvous points to prove when goroutines are blocked/unblocked.
2. Keep short watchdog timeouts only as deadlock safety nets, not as primary assertions.
3. Document the concurrency invariants each test validates.

Acceptance criteria

• Primary pass/fail logic does not depend on elapsed wall-clock thresholds.
• Tests remain stable across loaded CI runners.
• Existing behavioral guarantees are preserved.

Notes

This is test reliability hardening and complements issue #24 (stress coverage).