feat: enforce escrow funding on mission creation

[lishmanTech]

This PR introduces escrow funding enforcement for mission creation. closes #25
A mission can only be created if the creator fully funds the total reward upfront, ensuring participants are guaranteed payment before starting work.

Motivation

Participants need assurance that a mission is fully funded before contributing effort.
By locking rewards in escrow at creation time, we enforce a strong on-chain invariant:

If a mission exists, its rewards are already secured.

What’s Changed

Added escrow funding logic to create_mission

Full reward amount is calculated as:

reward_amount × max_participants

Tokens are transferred from the mission creator to the contract using soroban_sdk::token::Client

Mission creation fails atomically if:

Reward amount is zero or negative

Creator has insufficient token balance

Arithmetic overflow occurs

Updated tests to:

Deploy a real Stellar token contract

Mint balances for escrow testing

Validate failure when funds are insufficient

Verify escrow balance after mission creation

[Kaylahray]

Hi @lishmanTech 🙋‍♀️. Thank you for contributing. Please Kindly run this in quid-contract/ "cargo fmt --all -- --check" then to fix the error run "cargo fmt --all" . Then run "cargo clippy --all-targets --all-features -- -D warnings"

[Kaylahray]

Fixed @lishmanTech . Thanks