feat: integrate yield distribution with Soroban liquidity pools

[ChukwuebukaJohnDev]

Overview

This PR adds a Soroban Yield Distribution Integration system that deposits idle XLM/USDC from inheritance-plan vaults into Soroban liquidity pools, tracks yield token shares per plan, and enables real yield generation on testnet assets.

Related Issue

Closes #

Changes

🏦 Yield Pool Deposit Integration

• [MODIFY] contracts/inheritance-contract/src/lib.rs
  • Implements deposit_to_yield_pool(env: Env, pool_address: Address, amount: i128) using the Soroban token client.
  • Handles token approval and transfers idle XLM/USDC from the plan vault into the target Soroban liquidity pool.
  • Invokes existing yield_math.rs formulas to compute share entitlement during deposit.

📊 Per-Plan Yield Share Tracking

• [MODIFY] contracts/inheritance-contract/src/lib.rs
  • Adds persistent per-plan yield share accounting via StorageMap<(Address, Address), i128> mapped by (plan, pool_address).
  • Updates share balances on deposit so each plan’s yield is tracked independently and accurately.

✅ Test Coverage

• [ADD] #[cfg(test)] module in contracts/inheritance-contract/src/lib.rs
  • Covers deposit_to_yield_pool with mocked Soroban token/pool clients.
  • Verifies per-plan share balance updates and consistency with yield_math.rs.

Verification Results

cargo test -p inheritance-contract
✅ 18/18 passed

Testnet acceptance check:
✅ deposit_to_yield_pool executes via Soroban token client
✅ Idle XLM/USDC deposited into Soroban liquidity pool
✅ Yield token shares tracked per plan
✅ yield_math.rs share math verified against live pool result

Acceptance Criteria	Status
Deposit idle XLM/USDC into Soroban yield pools	✅ Cross-contract deposit implemented and testnet-verified
Yield token shares balance tracked per plan	✅ Per-plan storage map updated on every deposit
Real yield generation on testnet assets	✅ Verified against testnet XLM/USDC pool
yield_math.rs connected to live pool invocation	✅ Share math invoked from deposit_to_yield_pool

Closes #1143

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[ONEONUORA]

@ChukwuebukaJohnDev
Run cargo fmt --all -- --check and clippy also

[ONEONUORA]

@ChukwuebukaJohnDev
Pls update on this pr

[ONEONUORA]

@ChukwuebukaJohnDev
Pls update on this PR

[ONEONUORA]

@ChukwuebukaJohnDev
Pls fix conflict