feat(vault): add close_vault so an owner can remove a vault entry

[0dillon]

Summary

open creates a vault and withdraw takes funds out, but nothing removes the entry
(contracts/rent_vault/src/lib.rs). An owner who is finished with a target can drain the balance to
zero and still leaves a DataKey::Vault entry behind, which they keep paying rent on and which
open will refuse to replace.

Acceptance Criteria

• close_vault(env, target) exists, authorized by the vault owner.
• It refuses to close a vault with a non-zero balance, with its own error code in the 2xx
  range, so funds cannot be stranded.
• It removes the persistent entry rather than zeroing it.
• After closing, get_vault returns None and open succeeds for the same target.
• A #[contractevent] is emitted.
• Tests cover the happy path, the non-zero-balance rejection, and a close attempted by someone
  who is not the owner.

Tech Stack

Rust (edition 2021, toolchain 1.93.0 pinned in rust-toolchain.toml), soroban-sdk 27.0.4,
target wasm32v1-none. Build with stellar contract build, never cargo build. Tests are
#[cfg(test)] modules on Env::default(); run them with cargo test --all.