test: add seeded message delivery shuffling to QBFT tests

[diegomrsantos]

Problem, Evidence, and Context (Required)

• QBFT integration tests deliver messages to operators in a fixed order (1, 2, 3, ..., N), meaning they never explore what happens when messages arrive at different operators in different orders.
• Real networks don't deliver messages simultaneously or in a fixed order — message timing and arrival order vary per topology and load.
• Inspired by commonware-runtime's deterministic simulation approach: same seed = same interleaving = reproducible failures.

Change Overview (Required)

• QbftTester gains an optional seeded RNG that shuffles operator delivery order in process_network_message. When a seed is provided, each message broadcast delivers to operators in a different (but reproducible) shuffled order.
• TestContext::new_seeded() threads the seed through for ergonomic test construction.
• Three new multi-seed tests exercise basic consensus, f-faulty scenarios across committee sizes, and concurrent instances — each across multiple seeds.
• Existing tests are unchanged (pass None for seed, preserving fixed-order delivery).

Risks, Trade-offs, and Mitigations (Required)

• Test-only change, zero production code modified.
• Seeded shuffling is deterministic: if a seed fails in CI, re-running with that seed reproduces the exact same behavior locally.
• Existing tests preserve their original behavior.

Validation (Required)

• All 19 tests pass (16 existing + 3 new seeded tests).
• make cargo-fmt-check, make lint clean.
• New seeded tests cover: 10 seeds × basic consensus, 5 seeds × 4 committee sizes with faults, 10 seeds × concurrent instances.

Rollback (Required for behavior or runtime changes; optional otherwise)

N/A — test-only change, safe to revert.

Additional Info / Next Steps (Optional)

• This is the first step toward commonware-style deterministic testing. Future work could add clock abstraction to the QBFT orchestration layer, enabling full deterministic simulation of timing and scheduling interleavings.