ML-158: force single-line log output in production

Three-layer architecture:
- Logster v2 handles HTTP request and LiveView socket telemetry in logfmt format
- Custom formatter as safety net escaping any remaining embedded newlines
- Config flag (single_line_logging) gating Logster attachment to prod only

Dev environment retains multi-line format for readability.

test/music_library/logger/single_line_formatter_test.exs

@@ -0,0 +1,143 @@
+defmodule MusicLibrary.Logger.SingleLineFormatterTest do
+  use ExUnit.Case, async: true
+
+  alias MusicLibrary.Logger.SingleLineFormatter
+
+  @ts {{2024, 1, 15}, {14, 30, 45, 123}}
+
+  describe "environment configuration" do
+    test "single_line_logging is disabled in dev/test" do
+      refute Application.fetch_env!(:music_library, :single_line_logging)
+    end
+
+    test "default formatter is the standard pattern (not our custom module)" do
+      default_formatter = Application.get_env(:logger, :default_formatter, [])
+
+      refute match?({MusicLibrary.Logger.SingleLineFormatter, _}, default_formatter[:format])
+      assert is_binary(default_formatter[:format])
+    end
+
+    test "Phoenix.Logger is not disabled in dev/test" do
+      phoenix_logger = Application.get_env(:phoenix, :logger, true)
+      assert phoenix_logger
+    end
+  end
+
+  describe "format/4" do
+    test "replaces embedded newlines with escaped \\n" do
+      result =
+        SingleLineFormatter.format(:error, "line1\nline2\nline3", @ts, [])
+        |> IO.chardata_to_string()
+
+      assert result =~ "line1\\nline2\\nline3"
+      refute String.contains?(result, "\nline2")
+      assert String.ends_with?(result, "\n")
+    end
+
+    test "messages without newlines pass through unchanged" do
+      result =
+        SingleLineFormatter.format(:info, "hello world", @ts, [])
+        |> IO.chardata_to_string()
+
+      assert result =~ "hello world"
+      assert String.ends_with?(result, "\n")
+    end
+
+    test "handles iolist input" do
+      iolist = ~c"hello\nworld"
+
+      result =
+        SingleLineFormatter.format(:error, iolist, @ts, [])
+        |> IO.chardata_to_string()
+
+      assert result =~ "hello\\nworld"
+      assert String.ends_with?(result, "\n")
+    end
+
+    test "preserves metadata in output" do
+      result =
+        SingleLineFormatter.format(:info, "test message", @ts, request_id: "abc123")
+        |> IO.chardata_to_string()
+
+      assert result =~ "request_id=abc123"
+    end
+
+    test "handles empty messages" do
+      result =
+        SingleLineFormatter.format(:info, "", @ts, [])
+        |> IO.chardata_to_string()
+
+      assert String.ends_with?(result, "\n")
+      assert result =~ "[info]"
+    end
+
+    test "handles nil metadata" do
+      result =
+        SingleLineFormatter.format(:info, "msg", @ts, [])
+        |> IO.chardata_to_string()
+
+      assert result =~ "[info] msg"
+    end
+
+    test "output is a single line (no unescaped newlines in the message part)" do
+      result =
+        SingleLineFormatter.format(:error, "a\nb\nc", @ts, [])
+        |> IO.chardata_to_string()
+
+      lines = String.split(result, "\n", trim: true)
+      assert length(lines) == 1
+    end
+
+    test "includes level and timestamp" do
+      result =
+        SingleLineFormatter.format(:warning, "caution", @ts, [])
+        |> IO.chardata_to_string()
+
+      assert result =~ "[warning]"
+      assert result =~ "14:30:45.123"
+    end
+
+    test "handles multiple metadata keys" do
+      result =
+        SingleLineFormatter.format(:info, "msg", @ts, request_id: "req1", pid: self())
+        |> IO.chardata_to_string()
+
+      assert result =~ "request_id=req1"
+    end
+
+    test "pipe character in message is preserved" do
+      result =
+        SingleLineFormatter.format(:info, "status=200|duration=10ms", @ts, [])
+        |> IO.chardata_to_string()
+
+      assert result =~ "status=200|duration=10ms"
+    end
+
+    test "handles non-binary, non-list message via inspect fallback" do
+      result =
+        SingleLineFormatter.format(:info, %{key: "value"}, @ts, [])
+        |> IO.chardata_to_string()
+
+      assert result =~ "%{key: \"value\"}"
+    end
+
+    test "handles invalid iolist gracefully (UnicodeConversionError)" do
+      # Surrogate codepoint 0xD800 is not valid Unicode
+      result =
+        SingleLineFormatter.format(:error, [0xD800], @ts, [])
+        |> IO.chardata_to_string()
+
+      assert result =~ "[error]"
+      assert String.ends_with?(result, "\n")
+    end
+
+    test "unknown metadata keys are included via inspect" do
+      result =
+        SingleLineFormatter.format(:info, "msg", @ts, custom_key: "custom_val")
+        |> IO.chardata_to_string()
+
+      assert result =~ "custom_key="
+      assert result =~ "custom_val"
+    end
+  end
+end