fix(sdk): Add comprehensive retry logic with exponential backoff for network failures

[Kevin737866]

Description

Intelligent retry for transient errors.

Acceptance Criteria

• RetryConfig: maxRetries, baseDelayMs, maxDelayMs, backoffMultiplier
• Retryable: network timeout, 429, horizon 504, mempool full
• Non-retryable: invalid signature, insufficient funds, contract error
• Retry state events for UI feedback
• Tests: retry on timeout, no retry on auth, max exhausted, backoff timing

Files to Modify

• sdk/src/errors.ts
• sdk/src/index.ts
• sdk/src/constants.ts

[Emmanuel-Ugochukwu1]

@Emmanuel-Ugochukwu1 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hi Maintainer, i would love to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Emmanuel-Ugochukwu1 to this issue.

[IsaacMsheliaSamuel]

@IsaacMsheliaSamuel has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Please I would like to work on this issue.

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[frankosakwe]

@frankosakwe has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hello maintainer i would like to work on this issue

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[smithd2423-beep]

@smithd2423-beep has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

I can efficiently build the exponential backoff mechanism, filter retryable vs non-retryable errors according to the criteria, and write comprehensive unit tests to ensure stability.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @smithd2423-beep to this issue.

[Vincent6581]

@Vincent6581 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

I would like to work on this issue

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[drips-wave]

Congratulations, @Vincent6581! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

🧑‍💻 @Vincent6581: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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