Refactor tls_findings into smaller helper functions. (#28)

This change splits the large `tls_findings` function in `src/netsleuth/interpret.py` into three focused helper functions (`_check_tls_error`, `_check_tls_performance`, and `_check_tls_certificate`) to improve readability and maintainability.
Also fixes unused imports identified by ruff.

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

src/netsleuth/interpret.py

@@ -755,12 +755,10 @@ def _cpu_bound_ratio(tls_handshake_ms: float | None, tcp_rtt_ms: float | None) -
     return tls_handshake_ms / tcp_rtt_ms
 
 
-def tls_findings(results: list[TlsResult], t: Thresholds) -> list[Finding]:
-    findings: list[Finding] = []
-    for r in results:
-        if r.error:
-            findings.append(
-                Finding(
+def _check_tls_error(r: TlsResult) -> Finding | None:
+    if not r.error:
+        return None
+    return Finding(
         id=f"tls.unreachable.{r.label}",
         severity="warn",
         title=f"TLS handshake to {r.host} failed",
@@ -772,8 +770,10 @@ def tls_findings(results: list[TlsResult], t: Thresholds) -> list[Finding]:
         detail_ru=f"Не удалось установить TLS-соединение с {r.host}:{r.port}: {r.error}",
         advice_ru="Сервис может быть недоступен, либо порт заблокирован на пути к серверу.",
     )
-            )
-            continue
+
+
+def _check_tls_performance(r: TlsResult, t: Thresholds) -> list[Finding]:
+    findings: list[Finding] = []
     ratio = _cpu_bound_ratio(r.tls_handshake_ms, r.tcp_rtt_ms)
     if ratio is not None and ratio > t.tls_cpu_bound_ratio and (r.tls_handshake_ms or 0.0) >= 20.0:
         findings.append(
@@ -825,6 +825,11 @@ def tls_findings(results: list[TlsResult], t: Thresholds) -> list[Finding]:
                 advice_ru="Медленный TTFB при быстром рукопожатии указывает на обработку на стороне сервера, а не на сеть.",
             )
         )
+    return findings
+
+
+def _check_tls_certificate(r: TlsResult) -> list[Finding]:
+    findings: list[Finding] = []
     if r.pin_verdict == "mismatch":
         findings.append(
             Finding(
@@ -878,6 +883,18 @@ def tls_findings(results: list[TlsResult], t: Thresholds) -> list[Finding]:
     return findings
 
 
+def tls_findings(results: list[TlsResult], t: Thresholds) -> list[Finding]:
+    findings: list[Finding] = []
+    for r in results:
+        err = _check_tls_error(r)
+        if err:
+            findings.append(err)
+            continue
+        findings.extend(_check_tls_performance(r, t))
+        findings.extend(_check_tls_certificate(r))
+    return findings
+
+
 def prefix_findings(bench: PrefixBenchmark, t: Thresholds) -> list[Finding]:
     findings: list[Finding] = []
     if not bench.results: