Fix Stellar transaction replay with sequence number guard

[Vivian-04]

PR #540 Prevent Stellar transaction replay with sequence number guard

Description

This PR addresses issue #540 where concurrent submissions of identical transactions or slow RPC responses could cause sequence number conflicts, resulting in multiple confirmable duplicated transactions on-chain.

Changes

• Extracted the initially targeted transaction sequence inside _attemptSubmit.
• Added staleness check against the localSequenceTracker both before and after updating it from chain.
• If the target sequence is older than or equal to the tracked sequence, the system correctly halts and throws a Stale sequence number detected error instead of silently rebuilding and submitting the transaction.

Acceptance Criteria Met

• Local sequence number tracked after first account fetch.
• Sequence incremented on successful submission.
• Stale sequence numbers rejected with a clear error.
• Concurrent submissions handled safely.

Closes #540

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