Add NFT gate status checker

[Kingsman-99]

Labels: complexity: high

Implement checkNftGate(creatorAddress) in the SDK that queries the configured NFT gate contract and returns whether the given address holds a qualifying NFT. Should be called before createInvoice when the contract has an NFT gate configured.

Acceptance criteria:

• checkNftGate(address) returns { gated: boolean, hasNft: boolean, contractAddress: string | null }
• Returns { gated: false } when no NFT gate is configured
• Caches result for 30 seconds to avoid redundant RPC calls
• Exported with full TypeScript types

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[kokobutter-web]

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

I'm a full-stack developer and I'd like to take this issue.
I understand the requirements and will follow the existing project structure, keep the implementation clean, and include proper tests where applicable.

ETA: 1–3 days (depending on scope)

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Congratulations, @kokobutter-web! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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