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/csv.go

@@ -3,6 +3,7 @@ package internal
 import (
 	"encoding/csv"
 	"errors"
+	"fmt"
 	"io"
 	"os"
 	"strconv"
@@ -25,6 +26,7 @@ func ReadCsvRedirects(file string) (Redirects, error) {
 	if err != nil {
 		return nil, err
 	}
+	defer reader.Close()
 
 	csvReader := csv.NewReader(reader)
 
@@ -44,7 +46,7 @@ func ReadCsvRedirects(file string) (Redirects, error) {
 	// Read data
 	redirects := Redirects{}
 
-	for {
+	for line := 2; ; line++ {
 		row, err := csvReader.Read()
 		if err != nil {
 			if errors.Is(err, io.EOF) {
@@ -52,22 +54,20 @@ func ReadCsvRedirects(file string) (Redirects, error) {
 			}
 			return nil, err
 		}
-		if row == nil {
-			break
-		}
 
 		statusCode, err := strconv.Atoi(row[statusCol])
 		if err != nil {
-			return nil, err
+			return nil, fmt.Errorf("line %d: status code %q is not a number", line, row[statusCol])
+		}
+		if statusCode < 300 || statusCode > 399 {
+			return nil, fmt.Errorf("line %d: status code %d is not a redirect (must be 3xx)", line, statusCode)
 		}
 
-		redirect := &Redirect{
+		redirects = append(redirects, &Redirect{
 			From:       row[fromCol],
 			To:         row[toCol],
 			StatusCode: statusCode,
-		}
-
-		redirects = append(redirects, redirect)
+		})
 	}
 
 	return redirects, nil