feat: Add max slippage guard rails for strategy rebalance operations

[thebabalola]

Description

This PR resolves #561 by introducing max slippage guard rails and failure-safe behavior for strategy rebalance operations.

Changes Made:

• Added a new rebalance function to the YieldVault contract (vault/src/lib.rs). This allows an atomic divest-and-invest flow between two distinct strategies.
• Enforced min_divest_value to check against oracle manipulation or unexpected token deductions when withdrawing from the from_strategy.
• Enforced min_invest_value to verify that the target to_strategy accrues at least the expected equivalent valuation during a deposit.
• Included robust failure-safe boundaries: Since the rebalance steps are fully atomic, any slippage threshold breach (either during withdrawal or deposit) instantly reverts the entire transaction.
• Extended the VaultError enum with SlippageExceeded = 15.

This implementation mitigates the risk of value leakage when transitioning capital between connected yield strategies.

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