Clarify TLS scans without non-PQC probing

[leemeo3]

Summary

• encode nonpqc_algos as null when a TLS scan did not run non-PQC group probes
• add an info log that points users to --test-nonpqc-algos when no configured PQC groups are detected
• add unit coverage for the TLS non-PQC result encoding helper

Why

Issue #13 reports TLS scan output where the connection succeeds (error: null) but all algorithm lists are empty. I reproduced the same default-scan shape with www.openssl.org:443 and badssl.com:443.

The default TLS scan only probes configured PQC groups unless --test-nonpqc-algos is supplied. Returning nonpqc_algos: [] made that look like non-PQC groups were tested and none were found. Returning null keeps the existing optional field semantics and distinguishes "not tested" from an empty tested result.

Validation

• rustfmt --edition 2021 --check src/tls.rs
• cargo test
• cargo build
• ./target/debug/pqcscan tls-scan -t www.openssl.org:443 -o /tmp/pqcscan-openssl-after-default.json
• ./target/debug/pqcscan tls-scan -t www.openssl.org:443 --test-nonpqc-algos -o /tmp/pqcscan-openssl-after-nonpqc.json
• ./target/debug/pqcscan tls-scan -t pq.cloudflareresearch.com:443 -o /tmp/pqcscan-cloudflare-after.json

Notes

Full cargo fmt --check and cargo clippy --all-targets --all-features still fail on existing baseline issues outside the scope of this patch.

[leemeo3]

Rechecked this with a local OpenSSL TLS server restricted to classical groups (X25519:P-256), so the reproduction does not depend on an external endpoint.

On current origin/main, a default TLS scan against that local server reproduces the reported shape:

• error: null
• pqc_supported: false
• pqc_algos: []
• hybrid_algos: []
• nonpqc_algos: []

With --test-nonpqc-algos on origin/main, the same server reports SECP256R1 and X25519, confirming the server is reachable and has classical TLS groups.

On this PR branch, the same default scan returns nonpqc_algos: null and logs the --test-nonpqc-algos hint, while --test-nonpqc-algos still reports SECP256R1 and X25519.

Validation:

• cargo test -> pass (2 tests)
• rustfmt --edition 2021 --check src/tls.rs -> pass