fix: strengthen FROZEN boundary stress test semantics

[jverdicc]

Motivation

• Ensure the security-critical leakage budget boundary is strictly enforced and make the test explicitly validate that once the pool transitions to FROZEN, subsequent charges continue to be rejected as Err(EvidenceOSError::Frozen).

Description

• Tighten boundary_transition_from_active_to_frozen_has_no_off_by_one_leakage in crates/evidenceos-core/tests/stress_tests.rs by asserting a second post-freeze charge also returns Err(EvidenceOSError::Frozen) to validate the N+1 and N+2 semantics.

Testing

• Ran the targeted test with cargo test -p evidenceos-core --test stress_tests boundary_transition_from_active_to_frozen_has_no_off_by_one_leakage -- --nocapture, observed overflow_result = Err(Frozen), and the test passed.
• Attempted cargo test --workspace but the full workspace run did not complete within the environment runtime limits (targeted test coverage was used to validate the change).

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Codex Task