[Contract 11] Replace unchecked bToken addition with checked math

[sochima2]

close #158

Description
Replaced unchecked additions for user bTokens, total bTokens and legacy Blend balance with a helper fn checked_add_i128(env: &Env, lhs: i128, rhs: i128) -> i128 that uses panic_with_error!(env, Error::Overflow) on overflow. (file: contracts/src/lib.rs).
Introduced a typed contract error pub enum Error { Overflow, DeadlineExpired } and imported panic_with_error/contracterror to report failures consistently.
Added the missing remove_liquidity method to the SoroswapRouterTrait and provided a mock implementation used by withdraw tests so the withdrawal path compiles and runs in tests.
Minor formatting and test harness adjustments (use of env.register(..., ()), multiline storage calls) and added a regression test test_blend_supply_btoken_overflow_panics that seeds a b_tokens = i128::MAX position and asserts a subsequent supply panics instead of overflowing.

Testing
Ran cargo test in contracts/ and all tests passed (14 passed, 0 failed).
Ran cargo fmt --all -- --check and it passed.
Ran cargo clippy --all-targets -- -D warnings and no new warnings were introduced (clippy passed).

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