feat(soroban): implement get/set storage bytes subscript

[aryanbaranwal001]

Description

Adds support for reading or writing a particular byte of a bytes state variable on soroban by implementing previously stubbed functions of get_storage_bytes_subscript and set_storage_bytes_subscript with unimplemented! in soroban/target.rs.

The following contract now compiles to a valid WASM binary

// SPDX-License-Identifier: Apache-2.0
contract BytesSubscript {
    bytes public data;

    function set_data(bytes memory d) public {
        data = d;
    }

    function get_byte(uint32 i) public view returns (bytes1) {
        return data[i];
    }

    function set_byte(uint32 i, bytes1 v) public {
        data[i] = v;
    }
}

Key Changes

• codegen/mod.rs: added BytesGet and BytesPut variants to HostFunctions & their "b.6" / "b.5" name mappings.

• emit/soroban/mod.rs: added BytesGet and BytesPut arms to function_signature, and added BytesLen / BytesGet / BytesPut to the declare_externals host_functions list.

• emit/soroban/target.rs: implemented get_storage_bytes_subscript and set_storage_bytes_subscript.

Tests

• Added 8 tests in tests/soroban_testcases/bytes_subscript.rs covering read at every index, boundary writes, write isolation against unrelated bytes, sequential writes at the same index, and three out-of-the-bounds trap cases.

[aryanbaranwal001]

This PR is continuation of the work in #1908 and depends on it.