Add `subscribe()` token-address validation

[orunganiekan]

Complexity: High (200 points)

Description
subscribe() accepts any Address as token. If a non-contract address is passed, the token::Client::new() call will succeed but allowance() will fail at runtime with an opaque error.

Requirements and context

• Validate that the token address is a contract (not an account) before storing the subscription
• Use env.deployer().get_contract_id() or similar Soroban API
• Add ContractError::InvalidTokenAddress variant
• Add test: non-contract address as token panics with typed error

Suggested execution

git checkout -b fix/token-address-validation

• Edit contract/src/lib.rs → subscribe()
• Edit contract/src/errors.rs
• Add test

Acceptance criteria

• cargo test passes

[Swayymalcolm99]

@Swayymalcolm99 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hello maintainer. Can I take this, I'll deliver properly and on time

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[software321dev]

@software321dev has applied to work on this issue as part of the Stellar Wave Program's 5th 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)

ℹ️ Repo Maintainers: To accept this application, review their application or assign @software321dev to this issue.

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Congratulations, @software321dev! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

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[drips-wave]

This issue has been completed by @software321dev as part of the Stellar Wave Program's 5th Wave 🥳

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