Implement SDK request deduplication for identical concurrent calls

[Kingsman-99]

Overview

When multiple parts of the app call sdk.getInvoice(id) simultaneously for the same invoice, each triggers a separate RPC call. Add request deduplication so identical in-flight requests share a single network call.

Acceptance Criteria

• Concurrent calls with identical method + args share one in-flight request (promise deduplication)
• Deduplication window: the lifetime of the in-flight request — not a time window
• Second caller receives the same resolved/rejected value as the first
• Cache and deduplication are independent: deduplication applies even when cache is disabled
• sdk.getDedupStats() returns { deduped: number, total: number } for observability
• Deduplication opt-out per call: sdk.getInvoice(id, { dedupe: false }) for cases where fresh data is critical
• Works correctly with the retry layer — a retried request starts a new dedup window

[Turawa2]

@Turawa2 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I can work on the issue perfectly.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Turawa2 to this issue.

[coderolisa]

@coderolisa has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello maintainer let me work on this
please assign

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[ambermartin681]

@ambermartin681 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

hello dear maintainer i would love to work along side you on this task sir.

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[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #386)
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