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/console-log-appender.test.ts

@@ -0,0 +1,115 @@
+import { describe, test, expect } from "bun:test";
+import { ConsoleLogAppender, 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("ConsoleLogAppender", () => {
+  test("defaults threshold to ALL and formatter to flattenMessage", () => {
+    const appender = new ConsoleLogAppender();
+
+    expect(appender.threshold).toBe(LogLevel.ALL);
+    expect(appender.formatter).toBeDefined();
+  });
+
+  test("accepts custom threshold and formatter", () => {
+    const fmt = () => "custom";
+    const appender = new ConsoleLogAppender(LogLevel.ERROR, fmt);
+
+    expect(appender.threshold).toBe(LogLevel.ERROR);
+    expect(appender.formatter).toBe(fmt);
+  });
+
+  test("threshold of ALL (0) does not cause falsy guard to block messages", () => {
+    // ALL (0) is falsy in JS. The guard `if (this.threshold && msg.level < this.threshold)`
+    // evaluates `0 && ...` = 0 (falsy), so the if-block is NOT entered and
+    // messages proceed. This is correct behavior by accident — ALL means log everything.
+    const appender = new ConsoleLogAppender(LogLevel.ALL);
+
+    // Calling appendMessage should not throw for any level when threshold is ALL.
+    // We can't easily spy on console in Bun, but we can verify no errors are thrown.
+    for (const level of [
+      LogLevel.ALL,
+      LogLevel.TRACE,
+      LogLevel.DEBUG,
+      LogLevel.INFO,
+      LogLevel.WARN,
+      LogLevel.ERROR,
+      LogLevel.FATAL,
+    ]) {
+      expect(() => appender.appendMessage(makeMsg({ level }))).not.toThrow();
+    }
+  });
+
+  test("respects threshold by dropping messages below it", () => {
+    // We verify threshold works by checking the formatter is called (or not).
+    // When threshold is WARN, messages with INFO should not invoke the formatter.
+    let formatterCalls = 0;
+    const appender = new ConsoleLogAppender(LogLevel.WARN, () => {
+      formatterCalls++;
+      return "logs";
+    });
+
+    // This should be dropped before formatter is called
+    appender.appendMessage(makeMsg({ level: LogLevel.INFO }));
+    expect(formatterCalls).toBe(0);
+
+    // This should pass through
+    appender.appendMessage(makeMsg({ level: LogLevel.WARN }));
+    expect(formatterCalls).toBe(1);
+
+    // And higher levels too
+    appender.appendMessage(makeMsg({ level: LogLevel.ERROR }));
+    expect(formatterCalls).toBe(2);
+  });
+
+  test("does not throw for any valid log level", () => {
+    const appender = new ConsoleLogAppender();
+
+    for (const level of [
+      LogLevel.ALL,
+      LogLevel.TRACE,
+      LogLevel.DEBUG,
+      LogLevel.LOG,
+      LogLevel.INFO,
+      LogLevel.WARN,
+      LogLevel.ERROR,
+      LogLevel.FATAL,
+    ]) {
+      expect(() => appender.appendMessage(makeMsg({ level }))).not.toThrow();
+    }
+  });
+
+  test("handles string message with error/stacktrace", () => {
+    const appender = new ConsoleLogAppender();
+    const err = new Error("test error");
+
+    // Should not throw
+    expect(() =>
+      appender.appendMessage(makeMsg({ err, stacktrace: "trace" }))
+    ).not.toThrow();
+  });
+
+  test("handles object message formatter output", () => {
+    // The formatter can return an object (flattenMessage does),
+    // appendMessage should construct a summary and log it.
+    const appender = new ConsoleLogAppender(LogLevel.ALL, (msg) => ({
+      msg: "inner",
+      extra: 42,
+      level: LogLevel[msg.level],
+      scope: msg.scope,
+      stacktrace: "",
+      ts: msg.ts,
+    }));
+
+    expect(() => appender.appendMessage(makeMsg())).not.toThrow();
+  });
+});