test: add comprehensive unit tests for all appenders and logger

Tests added:
- log-service.test.ts: hierarchical logger creation and threshold propagation
- logger.test.ts: threshold inheritance, message propagation, falsy threshold
  bug coverage (LogLevel.ALL = 0), deferred messages, Error handling
- log-message.test.ts: parseLogLevel parsing, flattenMessage object/string modes
- console-log-appender.test.ts: threshold, formatter, all-level routing
- buffer-log-appender.test.ts: buffer append, threshold filtering, clearBuffer
- api-log-appender.test.ts: configuration defaults, threshold, auth token

Also fixes src/index.ts to export BufferLogAppender which was previously
missing from the barrel export.

test/buffer-log-appender.test.ts

@@ -0,0 +1,74 @@
+import { describe, test, expect } from "bun:test";
+import { BufferLogAppender, LogLevel, LogMessage } from "../src";
+
+function makeMsg(overrides: Partial<LogMessage> = {}): LogMessage {
+  return {
+    scope: "test-scope",
+    level: LogLevel.INFO,
+    msg: "test message",
+    stacktrace: "",
+    ts: new Date(),
+    ...overrides,
+  };
+}
+
+describe("BufferLogAppender", () => {
+  test("defaults to empty buffer and ALL threshold", () => {
+    const appender = new BufferLogAppender();
+
+    expect(appender.buffer).toEqual([]);
+    expect(appender.threshold).toBe(LogLevel.ALL);
+  });
+
+  test("accepts initial buffer", () => {
+    const existing: LogMessage[] = [makeMsg({ msg: "pre-existing" })];
+    const appender = new BufferLogAppender(existing);
+
+    expect(appender.buffer.length).toBe(1);
+    expect(appender.buffer[0].msg).toBe("pre-existing");
+  });
+
+  test("appends messages to buffer", () => {
+    const appender = new BufferLogAppender();
+
+    appender.appendMessage(makeMsg({ msg: "first" }));
+    appender.appendMessage(makeMsg({ msg: "second" }));
+
+    expect(appender.buffer.length).toBe(2);
+    expect(appender.buffer[0].msg).toBe("first");
+    expect(appender.buffer[1].msg).toBe("second");
+  });
+
+  test("respects threshold", () => {
+    const appender = new BufferLogAppender(undefined, LogLevel.WARN);
+
+    appender.appendMessage(makeMsg({ level: LogLevel.INFO, msg: "dropped" }));
+    appender.appendMessage(makeMsg({ level: LogLevel.WARN, msg: "kept" }));
+    appender.appendMessage(makeMsg({ level: LogLevel.ERROR, msg: "also kept" }));
+
+    expect(appender.buffer.length).toBe(2);
+    expect(appender.buffer[0].msg).toBe("kept");
+    expect(appender.buffer[1].msg).toBe("also kept");
+  });
+
+  test("threshold of ALL (0) does not cause falsy check to drop messages", () => {
+    const appender = new BufferLogAppender(undefined, LogLevel.ALL);
+
+    appender.appendMessage(makeMsg({ level: LogLevel.ALL, msg: "level all" }));
+    appender.appendMessage(makeMsg({ level: LogLevel.INFO, msg: "level info" }));
+
+    // Both should be kept — ALL (0) should not be treated as falsy
+    expect(appender.buffer.length).toBe(2);
+    expect(appender.buffer[0].msg).toBe("level all");
+    expect(appender.buffer[1].msg).toBe("level info");
+  });
+
+  test("clearBuffer empties the buffer", () => {
+    const appender = new BufferLogAppender();
+    appender.appendMessage(makeMsg({ msg: "to clear" }));
+    expect(appender.buffer.length).toBe(1);
+
+    appender.clearBuffer();
+    expect(appender.buffer.length).toBe(0);
+  });
+});