[WIP] Fix failing GitHub Actions job build-and-test

[Copilot]

• Root cause identified: soroban-env-host v21.2.1 uses ed25519-dalek >= 2.0.0 (no upper bound), and Cargo resolves it to 3.0.0, which changed the CryptoRng trait in a breaking way for rand_chacha RNGs used in testutils
• Fix: Create soroban-contracts/Cargo.toml workspace with [patch.crates-io] that replaces soroban-env-host with a local vendored copy
• Local vendor patch: soroban-contracts/vendor/soroban-env-host/ — copy of v21.2.1 with ed25519-dalek pinned to ">=2.0.0, <3"
• All 45 tests pass locally (cargo test from workspace)
• cargo fmt and cargo clippy pass cleanly