test: Horizon duplicate-hash chaos with idempotency

[Derekwalter999]

Description

Resolves #707 Stellar Horizon chaos: submit-endpoint duplicate-hash returns with idempotency assertion.

Context

When the Stellar submission worker encounters a timeout or is aggressively scaling, it might attempt to re-submit a transaction that has already been ingested by the network, resulting in a DUPLICATE status response from the Horizon endpoint. Alternatively, a client-side bug could trigger an actual true-duplicate submission.

Previously, StellarSubmissionService would throw an Errors.conflict upon receiving this status. This PR modifies the service to treat DUPLICATE responses as successes to prevent double-persisting, preserving idempotency without propagating a conflict error upstream.

Changes Made

• StellarSubmissionService Update: Modified the sendTransactionWithRetry block to catch DUPLICATE responses, immediately returning the pending/successful result to the caller instead of tossing a conflict exception.
• Metric Emission: Added the submission.duplicate.recovered counter metric via globalMetrics.increment() when a duplicate hash is gracefully recovered.
• Chaos Testing Harness: Created src/__tests__/chaos/horizonDuplicateHash.test.ts to assert against:
  • First-attempt duplicates (Retry recovery scenarios where the transaction was already successful).
  • True-duplicates (Simulating a client bug resulting in a double-submit).
  • Concurrent duplicate submissions (Two decoupled workers racing to submit the exact same transaction hash; one gets pending, one gets duplicate, but they both accurately coalesce and resolve gracefully).

Security & Correctness Assumptions Validated

• The cache size and deduplication sets are appropriately bypassed and checked.
• No duplicate database/ledger commits occur downstream because the service normalizes the DUPLICATE response as a standard, already processed success.
• Minimum 95% test coverage maintained.

Checklist

• Code successfully simulates and handles Horizon returning a duplicate hash.
• Includes both retry recovery and true-duplicate tests.
• Asserts concurrency coalescing (single ledger outcome).
• The submission.duplicate.recovered metric counter is actively emitted.
• Test suite executes without failure.

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