QA-BitVMX: Added tests to cover protocol configuration

[andrelo86]

Add config loading test coverage

Introduces config_test.rs with 4 tests covering Config::new():

1. Load development config — BITVMX_ENV=development parses successfully and validates builder.ecdsa_sighash_type and rpc.network
2. Default env fallback — unset BITVMX_ENV falls back to development correctly
3. Unknown env returns error — nonexistent environment returns ConfigError::ConfigFileError
4. All development fields — exhaustive validation of every readable field in development.json

• A static Mutex serializes env-var mutation across tests to prevent race conditions in parallel test runs.

• _guard explanation
  Here guard is held intentionally for its Drop side effect.
  _guard is the established Rust idiom for this exact pattern — its documented in The Rust Reference as suppress the warning for a binding kept alive for its destructor.