feat: Implement Platform Fee Configuration

[7maylord]

Summary
This PR adds the get_platform_fee_bps and set_platform_fee_bps feature and resolves a significant number of pre-existing bugs that prevented the contract from compiling and the test suite from running correctly.

New Feature:

• get_platform_fee_bps(env) -> u32 — reads the platform fee (in basis points) from instance storage; defaults to 0 if unset.
• set_platform_fee_bps(env, new_bps: u32) -> Result<(), Error> — updates the fee; restricted to the admin via require_auth(), validates the range, and emits a PlatformFeeUpdated(old, new) event.
• validate_platform_fee_bps(bps) helper — rejects any value above PLATFORM_FEE_MAX_BPS = 1000 (10%) with Error::InvalidFeeBps.
• init() now validates platform_fee_bps at initialization time using the same helper.
• Added 9 New Tests for the Platform Fee getter?setter

closes #46

[ahmadogo]

Hii @7maylord Please resolve Conflict

[7maylord]

@ahmadogo Resolved, please review.

[ahmadogo]

@7maylord One more Conflict

sorry

[7maylord]

Done @ahmadogo