feat(splitter): implement bounded cascade distribution helper

[Opulencechuks]

Closes #49

Pull Request Description

Draining a nested split tree currently needs one distribute call per node. This change introduces a distribute_cascade function that distributes a parent split and recursively distributes any freshly-credited direct children (and their children, etc.) in a single call.

The cascade is bounded by a max_depth parameter up to MAX_CASCADE_DEPTH (5) to protect against Soroban gas limit exhaustion.

A private helper distribute_node is extracted to reuse the balance-draining logic. Unit tests cover a two-level tree, depth bounds, and maximum depth limits.

What this changes

• Refactoring & Helpers: Extracted the core balance-draining logic into a private distribute_node function. Added a recursive helper distribute_recursive.
• New API Endpoint: Implemented distribute_cascade(env: Env, id: u64, token: Address, max_depth: u32) -> Result<i128, Error> on the Splitter contract.
• Constant Depth Bound: Added MAX_CASCADE_DEPTH (5) to limit the recursion and prevent stack/gas exhaustion.
• Typed Error: Added a new typed error Error::MaxDepthExceeded (Code 13) when the specified recursion depth exceeds the allowed maximum.
• Comprehensive Tests: Added unit tests covering basic two-level cascade routing, depth-0 boundary behavior, and max depth limits validation.

Why
Draining nested split trees previously required a separate external distribute invocation for each individual node in the tree. This cascade helper improves UX by allowing client applications to route and drain multiple levels of a tree in a single, atomic on-chain call.

Checklist

• cargo fmt --all and cargo clippy --all-targets -- -D warnings pass
• cargo test passes
• New behavior has tests
• Linked issue, if there is one

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[Spagero763]

@Opulencechuks kindly resolve conflicts

[Spagero763]

Heads up: this looks like the same feature as #284 (both titled "implement bounded cascade distribution helper"), and equivalent functionality (distribute_cascade / distribute_all_tokens) already merged to main via #285. Your contract build is also failing on a missing Error::NoPendingTransfer variant, a side effect of that merge renumbering the error enum. Given the feature's already on main, you may want to check #285 and either close this or find something additive on top of it rather than fixing the compile error.

[Spagero763]

Closing as duplicate/redundant — distribute_cascade already shipped via #285 (closing #49). Thanks for the contribution, but this PR's diff would now conflict with the existing implementation. Feel free to pick up something else from the open issues.