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/logger.test.ts

@@ -0,0 +1,104 @@
+import { describe, test, expect } from "bun:test";
+import { Logger, LogLevel, LogMessage } from "../src";
+
+describe("Logger", () => {
+  test("uses parent threshold when no threshold is set", () => {
+    const parent = new Logger("parent", undefined, LogLevel.WARN);
+    const child = new Logger("child", parent);
+
+    // Should not log below WARN (from parent)
+    const appender = { threshold: LogLevel.ALL, messages: [] as LogMessage[], appendMessage(msg: LogMessage) { this.messages.push(msg); } };
+    child.appenders = [appender];
+
+    child.info("should be dropped");
+    child.warn("should be logged");
+
+    expect(appender.messages.length).toBe(1);
+    expect(appender.messages[0].msg).toBe("should be logged");
+  });
+
+  test("overrides parent threshold when own threshold is set", () => {
+    const parent = new Logger("parent", undefined, LogLevel.WARN);
+    const child = new Logger("child", parent, LogLevel.ERROR);
+
+    const appender = { threshold: LogLevel.ALL, messages: [] as LogMessage[], appendMessage(msg: LogMessage) { this.messages.push(msg); } };
+    child.appenders = [appender];
+
+    child.warn("should be dropped");
+    child.error("should be logged");
+
+    expect(appender.messages.length).toBe(1);
+    expect(appender.messages[0].msg).toBe("should be logged");
+  });
+
+  test("propagates messages to parent appenders", () => {
+    const parent = new Logger("parent", undefined, LogLevel.ALL);
+    const parentAppender = { threshold: LogLevel.ALL, messages: [] as LogMessage[], appendMessage(msg: LogMessage) { this.messages.push(msg); } };
+    parent.appenders = [parentAppender];
+
+    const child = new Logger("child", parent);
+    const childAppender = { threshold: LogLevel.ALL, messages: [] as LogMessage[], appendMessage(msg: LogMessage) { this.messages.push(msg); } };
+    child.appenders = [childAppender];
+
+    child.info("hello");
+
+    expect(childAppender.messages.length).toBe(1);
+    expect(parentAppender.messages.length).toBe(1);
+  });
+
+  test("threshold of ALL (0) is not treated as falsy", () => {
+    // This is the threshold bug: `this.threshold &&` treats 0 (LogLevel.ALL) as falsy.
+    // When threshold is explicitly set to LogLevel.ALL (0), messages at ALL level
+    // should still pass through.
+    const logger = new Logger("test", undefined, LogLevel.ALL);
+    const appender = { threshold: LogLevel.ALL, messages: [] as LogMessage[], appendMessage(msg: LogMessage) { this.messages.push(msg); } };
+    logger.appenders = [appender];
+
+    logger.info("should be logged at ALL threshold");
+
+    expect(appender.messages.length).toBe(1);
+    expect(appender.messages[0].msg).toBe("should be logged at ALL threshold");
+  });
+
+  test("handles deferred messages", () => {
+    const logger = new Logger("test", undefined, LogLevel.ALL);
+    const appender = { threshold: LogLevel.ALL, messages: [] as LogMessage[], appendMessage(msg: LogMessage) { this.messages.push(msg); } };
+    logger.appenders = [appender];
+
+    logger.info(() => "deferred result");
+
+    expect(appender.messages.length).toBe(1);
+    expect(appender.messages[0].msg).toBe("deferred result");
+  });
+
+  test("handles Error objects", () => {
+    const logger = new Logger("test", undefined, LogLevel.ALL);
+    const appender = { threshold: LogLevel.ALL, messages: [] as LogMessage[], appendMessage(msg: LogMessage) { this.messages.push(msg); } };
+    logger.appenders = [appender];
+
+    const err = new Error("boom");
+    logger.error(err);
+
+    expect(appender.messages.length).toBe(1);
+    expect(appender.messages[0].msg).toBe("Error: boom");
+    expect(appender.messages[0].err).toBe(err);
+  });
+
+  test("log methods exist for all levels", () => {
+    const logger = new Logger("test", undefined, LogLevel.ALL);
+    const appender = { threshold: LogLevel.ALL, messages: [] as LogMessage[], appendMessage(msg: LogMessage) { this.messages.push(msg); } };
+    logger.appenders = [appender];
+
+    logger.trace("trace msg");
+    logger.debug("debug msg");
+    logger.log("log msg");
+    logger.info("info msg");
+    logger.warn("warn msg");
+    logger.error("error msg");
+    logger.fatal("fatal msg");
+
+    expect(appender.messages.length).toBe(7);
+    expect(appender.messages[0].level).toBe(LogLevel.TRACE);
+    expect(appender.messages[6].level).toBe(LogLevel.FATAL);
+  });
+});