[Chore] Collapse deposit()'s two sequential token transfers into one

[collinsezedike]

Summary

MeridianVault::deposit transfers USDC from the caller to the vault, then immediately transfers the same amount from the vault to the adapter, as two separate TokenClient::transfer calls.

Motivation

The intermediate vault-owned balance is never used for anything between the two transfers; it exists for exactly one instruction. Each transfer is a separate cross-contract call with its own instruction and resource cost on Soroban, so this doubles the token-transfer overhead of every deposit for no behavioral benefit.

Proposed Solution

Transfer USDC directly from the caller to the adapter in one call, since the adapter address is already known before any transfer happens.

Scope

Field	Value
Area	Contracts
Protocol affected	Both
Network	testnet, mainnet
Breaking change?	No

Alternatives Considered

Leaving it as-is: works correctly today, but pays unnecessary transaction cost on every deposit.

Acceptance Criteria

• deposit performs a single caller -> adapter transfer instead of caller -> vault -> adapter
• Existing deposit tests still pass unmodified (behavior is identical, only the transfer path changes)
• cargo clippy --all-targets -- -D warnings && cargo test pass

Additional Context

packages/contracts/vault/src/lib.rs, deposit function, the two TokenClient::new(&env, &usdc).transfer(...) calls.

[Ayinkx1]

Hi i would like to work on this issue please assign to me
I can do it perfectly

[Chigybillionz]

Hi team

I'd like to work on this issue.

I'll update the "deposit" flow to perform a single "caller -> adapter" USDC transfer instead of the current "caller -> vault -> adapter" path, keeping the existing behavior unchanged while reducing unnecessary transfer overhead. I'll verify all existing deposit tests continue to pass and ensure "cargo clippy" and "cargo test" complete cleanly.

GitHub: https://github.com/chiggybillionz