feat(stake-vault): add get_multiplier view function for cross-contrac…

[ajulaybeeb]

Added the get_multiplier view function to the StakeVault contract. This cross-contract view function returns a multiplier based on the amount of USDC staked by a user, and is used by the RewardPool and QuestEngine to calculate boosted payout amounts.

Changes

• Implemented get_multiplier(env: Env, user: Address) -> u32 in contracts/stake-vault/src/lib.rs.
• Created basic unit tests in contracts/stake-vault/src/test.rs to verify the staking threshold logic.
• Added contracts/stake-vault to the workspace members in Cargo.toml.

Acceptance Criteria Verified

• Returns 200 for stakes >= 500 USDC.
• Returns 120 for stakes between 100 and 499 USDC.
• Returns 100 for users with no stake or < 100 USDC.
  Depends on: [StakeVault] Stake USDC for multipliers
  Closes [StakeVault] Get user reward multiplier (view) #48

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