chore: keep test-env out of the default features

[otavio]

test-env gates the updatehub-setup-mock-env binary and the updatehub::tests helpers the integration tests drive the agent with. It was on by default, so anyone who follows the README and runs cargo build --release compiles 18 crates they will never run — mockito and its HTTP mock server among them — and pays for them on every build. Nobody should have to remember --no-default-features to get the plain agent.

The change

1. default = ["test-env"] becomes default = [].
2. crate::tests and crate::firmware::tests answer to cfg(test) as well, so the unit tests keep reaching them with no feature at all.
3. The three integration test targets gain required-features = ["test-env"]. They call updatehub::tests::TestEnvironment and mockito, and cargo builds the library without cfg(test) for an integration test, so the feature is the only way in. cargo test skips them; cargo test --all-features runs them.

async-ctrlc and mockito stay optional rather than move to dev-dependencies, because src/setup_mock_env.rs is a binary and cargo gives dev-dependencies to tests, examples and benches only.

Crates the default build no longer pulls

assert-json-diff, async-ctrlc, colored, ctrlc, getrandom, lock_api, mockito, nix, parking_lot, parking_lot_core, ppv-lite86, rand, rand_chacha, rand_core, scopeguard, serde_urlencoded, similar, zerocopy.

The stripped binary loses only 8 KiB (4855400 → 4847208 bytes), because the linker already dropped what the agent never called. The build graph is where this is paid, along with the supply-chain surface of a production build.

CI

No change needed. Every step already passes --all-features, so nothing loses coverage. The Check build with the default features step now covers the feature set users really get.

Verification

Command	Result
cargo check --locked --all --bins --examples --tests	clean
cargo check --locked --all --bins --examples --tests --all-features	clean
cargo clippy --locked --all-features --all --tests -- -D clippy::all	clean
cargo test	145 pass, 0 fail
cargo test --all --all-features --no-fail-fast	168 pass, 0 fail