C++ Unit Test Environment Decoupling & Refactoring

### What

Child of #49

**Description:**
Currently, C++ unit tests handle PKCS#11 token initialization inconsistently and fail when the Rust backend is selected:
1. test_pkcs11_multi_token.cpp and test_pkcs11_provider.cpp use unmaintainable #ifndef USE_RUST_PKCS11 preprocessor macros to skip token initialization entirely.
2. test_pkcs11_key_handler.cpp relies on SofthsmTestFixture. This fixture hardcodes the environment setup for SoftHSM (e.g., setting SOFTHSM2_CONF). When linked against the Rust backend, the test crashes because the Rust backend's required environment variable (CRYPTOKI_STORE) is never set, causing C_InitToken to fail.

We need to completely remove all #ifndef USE_RUST_PKCS11 preprocessor macro blocks and refactor the SofthsmTestFixture. By abstracting the test environment setup, selecting the PKCS#11 backend at Bazel build-time will automatically inject the correct environment variables and setup logic (SoftHSM vs. Rust), keeping the generic C++ test source files completely clean, decoupled, and vendor-neutral

This resolved the issue discussed here https://github.com/eclipse-score/inc_security_crypto/pull/49#:~:text=score/crypto/src/daemon,for%20test%20adaptation%20based

### Acceptance Criteria (DoD)

[ ]Zero Macros: Absolutely zero #ifndef USE_RUST_PKCS11 or #ifdef preprocessor blocks remain inside test_pkcs11_multi_token.cpp and test_pkcs11_provider.cpp.
[ ] Fixture Decoupling: The shared test fixture (SofthsmTestFixture) is replaced with a polymorphic interface that sets the correct environment variables (SOFTHSM2_CONF vs CRYPTOKI_STORE) before calling C_InitToken.

Start with test_pkcs11_multi_token.cpp, test_pkcs11_provider.cpp, and test_pkcs11_key_handler.cpp, then trace SofthsmTestFixture and the Bazel test setup. Identify how the selected backend supplies SOFTHSM2_CONF or CRYPTOKI_STORE before C_InitToken. Done means the shared fixture is replaced by a polymorphic environment setup and the two named tests contain no USE_RUST_PKCS11 preprocessor blocks.

Written by the indexing model from the issue text.