count links as a fixed length against the tweet limit

Twitter rewrites every URL to a t.co address of a fixed 23 characters,
so counting the raw characters rejected messages Twitter would have
accepted: 100 characters of text plus a 50 character YouTube link
counted as 150 and was refused, where Twitter sees 123.

The punctuation trimmed off the end of a link keeps counting as ordinary
text. Only links written with a scheme are recognised; Twitter also
shortens bare hosts like example.com/foo, but detecting those without
mistaking Node.js for a link is not worth it here, and counting them in
full errs toward refusing rather than posting something too long.

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

main_test.go

@@ -6,8 +6,10 @@ import (
 	"image/color"
 	"image/jpeg"
 	"math/rand"
+	"strings"
 	"testing"
 	"tweetdistributor/output"
+	"unicode/utf8"
 )
 
 // noiseJPEG returns a JPEG image of random noise, which compresses poorly
@@ -28,6 +30,42 @@ func noiseJPEG(t *testing.T, width, height int) []byte {
 	return buf.Bytes()
 }
 
+func TestTweetLength(t *testing.T) {
+	tests := []struct {
+		name    string
+		content string
+		want    int
+	}{
+		{"no link", "こんにちは", 5},
+		{"long link", "https://www.youtube.com/watch?v=dQw4w9WgXcQ&list=PLabcdefghijklmnop", urlLength},
+		{"short link", "https://a.jp", urlLength},
+		{"link with text", "みてね " + strings.Repeat("x", 10) + " https://youtu.be/dQw4w9WgXcQ", 4 + 10 + 1 + urlLength},
+		{"two links", "https://a.jp https://b.jp", urlLength*2 + 1},
+		{"trailing punctuation still counts", "https://a.jp。", urlLength + 1},
+		{"multibyte path", "https://ja.wikipedia.org/wiki/日本語", urlLength},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := tweetLength(tt.content); got != tt.want {
+				t.Errorf("tweetLength(%q) = %d, want %d", tt.content, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestTweetLengthAcceptsMessageOnlyLinksMakeTooLong(t *testing.T) {
+	// A message that the raw rune count would reject but Twitter accepts.
+	content := strings.Repeat("あ", 100) + " https://www.youtube.com/watch?v=dQw4w9WgXcQ&t=120s"
+
+	if raw := utf8.RuneCountInString(content); raw <= maxTweetLength {
+		t.Fatalf("test message should be %d raw characters, got %d", maxTweetLength+1, raw)
+	}
+	if got := tweetLength(content); got > maxTweetLength {
+		t.Errorf("tweetLength = %d, want it to fit within %d", got, maxTweetLength)
+	}
+}
+
 func TestShrinkImageLarge(t *testing.T) {
 	data := noiseJPEG(t, 3000, 3000)
 	if len(data) <= maxImageBytes {