Make transaction sequence allocation restart-safe

[Emelie-Dev]

Summary

This PR makes transaction sequence allocation restart-safe.

The implementation ensures that transaction sequences remain consistent across retries, concurrent requests, process restarts, timeouts, and partial failures, preventing duplicate sequences, conflicting state, and unrecoverable transaction state.

Changes

• Made transaction sequence allocation restart-safe across process restarts.
• Ensured sequence allocation is safe under concurrent requests/workers.
• Prevented duplicate or conflicting sequence assignments during retries and duplicate submissions.
• Added explicit handling for partial failures, timeouts, and retry scenarios.
• Preserved existing authorization, correctness, compatibility, and CI safeguards.
• Added focused regression tests covering duplicate, concurrent, failure, timeout, restart, and recovery paths.

Correctness & Recovery

The sequence allocation flow now guarantees that a retry or duplicate submission cannot produce conflicting transaction state or duplicate side effects.

Sequence state is persisted in a way that allows the operation to recover correctly after a process restart or partial failure, rather than relying on in-memory state that can be lost when the application stops.

Concurrent workers are also prevented from allocating conflicting sequences through the existing persistence guarantees.

Tests

Focused regression coverage was added for:

• Duplicate submissions / retries.
• Concurrent sequence allocation.
• Process restart and recovery.
• Timeout and partial-failure scenarios.
• Retry exhaustion and subsequent recovery.
• Prevention of duplicate side effects and conflicting transaction state.

Validation

The repository's applicable formatting, lint, build, and test commands were run.

Any unrelated pre-existing failures are reported separately and are not introduced by this change.

Related Issue

Closes #1186

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