Add `get_token_decimals` SDK method

[Levi-Ojukwu]

Description:

The contract exposes get_token_decimals(token: Address) (at lib.rs:430-434) that returns the decimal precision for a registered token (e.g., 6 for USDC, 7 for XLM). This is critical for correctly formatting token amounts in UI displays.

The contract implementation:

pub fn get_token_decimals(env: Env, token: Address) -> Option<u32> {
    env.storage()
        .persistent()
        .get(&crate::storage::DataKey::TokenDecimals(token))
}

The SDK method should:

1. Accept a token address parameter
2. Call the contract's get_token_decimals view
3. Return number | null (null if token is not registered)
4. Handle the Stellar address type conversion

Why it matters: Displaying token amounts correctly requires knowing the decimal precision. Without this, 1 USDC (10,000,000 stroops) could be displayed as 10000000 instead of 1.00.

Acceptance Criteria:

• Create a function in sdk/src/methods/ that calls get_token_decimals
• Accept a Stellar address string parameter
• Return Promise<number | null>
• Add tests with mock client
• Export from sdk/src/index.ts

Relevant Files: contracts/invoice_liquidity/src/lib.rs:430-434, sdk/src/index.ts

---

[GBOYEE]

Hey! I can pick this up. The task is adding a get_token_decimals SDK method. I'll implement it alongside the existing SDK methods (following the current SDK module pattern) and add a usage note to the docs. I'll get a PR up soon. Mind if I take this on?

[Robotron2]

I'd like to work on this issue.

I'll add a typed SDK method that wraps the contract's get_token_decimals view, handling Stellar address conversion and returning a Promise<number | null> for registered and unregistered tokens. I'll also export it through the SDK entry point and add mock client tests to verify the expected behavior and return types.

I've worked with Soroban SDKs and contract integration layers before, so I'm comfortable implementing this in a way that's consistent with the existing SDK patterns. I should be able to have a tested PR ready within a day.

[grantfox-oss]

🦊 GrantFox — @Robotron2 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #411)
2. Your PR will be reviewed by the Invoice-Liquidity-Network maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@Robotron2's PR #502 was approved and merged by @Nursca.

🏆 @Robotron2: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (484 total points). Track your full progress on GrantFox.

👏 Great work, @Robotron2! Keep contributing to Invoice-Liquidity-Network.