reject non-3xx status codes; close the CSV file; add tests

A row with a status code outside 3xx (e.g. "0" from a typo) was accepted
at load and then panicked net/http's WriteHeader on the first matching
request. ReadCsvRedirects now rejects anything that isn't 300-399, with a
line number in the message, and reports a non-numeric code clearly.

Also defer reader.Close() (the file was left open for the process
lifetime) and drop the unreachable "row == nil" check.

Adds internal/{csv,redirect,http}_test.go covering CSV parsing (valid,
reordered headings, header-only, missing heading, non-numeric and non-3xx
status, ragged row, missing file), Redirects.Find / Redirect.Match, and
ServeHTTP (redirect, 404, query string dropped).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/http_test.go

@@ -0,0 +1,56 @@
+package internal
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/rs/zerolog"
+)
+
+func newTestServer(rs Redirects) *HttpServer {
+	return &HttpServer{
+		CLI:       &CLI{},
+		Log:       zerolog.Nop(),
+		Redirects: rs,
+	}
+}
+
+func TestServeHTTP(t *testing.T) {
+	srv := newTestServer(Redirects{
+		{From: "/old", To: "https://example.com/new", StatusCode: 301},
+	})
+
+	t.Run("known path redirects with Location", func(t *testing.T) {
+		rec := httptest.NewRecorder()
+		srv.ServeHTTP(rec, httptest.NewRequest(http.MethodGet, "/old", nil))
+
+		if rec.Code != 301 {
+			t.Errorf("status = %d, want 301", rec.Code)
+		}
+		if loc := rec.Header().Get("Location"); loc != "https://example.com/new" {
+			t.Errorf("Location = %q", loc)
+		}
+	})
+
+	t.Run("unknown path is 404 with no Location", func(t *testing.T) {
+		rec := httptest.NewRecorder()
+		srv.ServeHTTP(rec, httptest.NewRequest(http.MethodGet, "/nope", nil))
+
+		if rec.Code != http.StatusNotFound {
+			t.Errorf("status = %d, want 404", rec.Code)
+		}
+		if loc := rec.Header().Get("Location"); loc != "" {
+			t.Errorf("Location = %q, want empty", loc)
+		}
+	})
+
+	t.Run("query string is dropped (current behaviour)", func(t *testing.T) {
+		rec := httptest.NewRecorder()
+		srv.ServeHTTP(rec, httptest.NewRequest(http.MethodGet, "/old?ref=x", nil))
+
+		if loc := rec.Header().Get("Location"); loc != "https://example.com/new" {
+			t.Errorf("Location = %q", loc)
+		}
+	})
+}