stellar: add retry logic for tx_bad_seq, tx_insufficient_fee, and net…

[CASHBUDDIE]

…work timeouts

## Description

Add robust retry logic to Stellar transaction submission to handle transient Horizon errors: refresh sequence on `tx_bad_seq`, bump fee on `tx_insufficient_fee`, and retry network timeouts with exponential backoff.

## Related Issue

Fixes #57  

## Type of Change

- [x] Bug fix
- [ ] New feature
- [ ] Documentation update
- [ ] Code refactoring
- [ ] Performance improvement

## Changes Made

- Implemented intelligent retrying inside `submitTransaction` in [src/stellar/transactions.ts](src/stellar/transactions.ts#L1-L400):
  - On `tx_bad_seq`: calls `updateFeePayerSequence()` (attempts to refresh fee payer sequence), re-parses the envelope and retries submission (bounded by network retry limit).
  - On `tx_insufficient_fee`: when the envelope is a `FeeBumpTransaction`, rebuilds the fee-bump with a 20% higher total fee (computes new base fee from charged operation count), re-signs with the fee payer and retries up to 3 times.
  - On network timeouts / server 5xx / connection resets: retries with exponential backoff (base 200ms, doubled each attempt) up to 5 attempts.
  - Other structured Stellar errors return immediately with the Horizon transaction result code (no retry).
- Added small local helpers in `submitTransaction`: `isNetworkError` and `stellarTxCode` to centralize error classification.
- Preserved existing fee-limit validation (`validateEnvelopeFeeLimit`) and fee-payer sequence update after successful submission.
- Minimal, focused change: no public API changes; only implements more resilient submission behavior.

## Testing

How did you test these changes?

- Unit tests: The repo includes existing tests that exercise fee-bump builds and submission behavior (`src/stellar/__tests__/transactions.test.ts`). I validated logic by running inspections against these tests and by reasoning through expected flows for each error case.
- Local run instructions (recommended):
```bash
# Install deps
npm install

# Run all tests
npm test

# Or run just the transactions tests
npm test -- transactions.test.ts -i --runInBand

• Note: In this editing environment I attempted to run the test command but npm was unavailable; please run the commands above locally or in CI to verify behavior fully.
• Suggested additional test cases (if you want to add more):
  • Mock server response that returns tx_bad_seq and verify that updateFeePayerSequence is called and the submission is retried.
  • Mock tx_insufficient_fee on a FeeBumpTransaction and verify the fee is increased by ~20% and retried up to 3 times.
  • Simulate network timeouts / 500 responses and verify exponential backoff occurs up to 5 attempts.
  • Verify other Horizon error codes (e.g., tx_failed) are returned immediately without retries.

Checklist

• Code follows project style
• Self-reviewed my code
• Commented complex code
• Updated documentation
• No new warnings
• Added tests (if applicable)

N/A

Additional Notes

• Branch: fix/stellar-retry-transaction-submission
• Commit: stellar: add retry logic for tx_bad_seq, tx_insufficient_fee, and network timeouts
• File changed: src/stellar/transactions.ts
• The change aims to make transaction submission resilient to transient Horizon conditions while keeping strict behavior for non-retryable errors. Please run the test suite in your environment / CI and I can follow up with any adjustments or add targeted unit tests if you want.

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