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/log-message.test.ts

@@ -0,0 +1,72 @@
+import { describe, test, expect } from "bun:test";
+import { parseLogLevel, LogLevel, flattenMessage, LogMessage } from "../src";
+
+describe("parseLogLevel", () => {
+  test("parses known level strings case-sensitively", () => {
+    expect(parseLogLevel("ALL")).toBe(LogLevel.ALL);
+    expect(parseLogLevel("TRACE")).toBe(LogLevel.TRACE);
+    expect(parseLogLevel("DEBUG")).toBe(LogLevel.DEBUG);
+    expect(parseLogLevel("INFO")).toBe(LogLevel.INFO);
+    expect(parseLogLevel("WARN")).toBe(LogLevel.WARN);
+    expect(parseLogLevel("ERROR")).toBe(LogLevel.ERROR);
+    expect(parseLogLevel("FATAL")).toBe(LogLevel.FATAL);
+  });
+
+  test("returns default level for unknown strings", () => {
+    expect(parseLogLevel("BOGUS")).toBe(LogLevel.INFO);
+    expect(parseLogLevel("")).toBe(LogLevel.INFO);
+  });
+
+  test("accepts custom default level", () => {
+    expect(parseLogLevel("BOGUS", LogLevel.WARN)).toBe(LogLevel.WARN);
+  });
+});
+
+describe("flattenMessage", () => {
+  test("string message includes all fields", () => {
+    const msg: LogMessage = {
+      scope: "my-scope",
+      level: LogLevel.WARN,
+      msg: "a warning",
+      ts: new Date("2024-01-01T00:00:00Z"),
+      stacktrace: "",
+    };
+
+    const flat = flattenMessage(msg);
+
+    expect(flat.scope).toBe("my-scope");
+    expect(flat.level).toBe("WARN");
+    expect(flat.msg).toBe("a warning");
+  });
+
+  test("object message promotes fields, drops reserved keys", () => {
+    const msg: LogMessage = {
+      scope: "my-scope",
+      level: LogLevel.INFO,
+      msg: { foo: "bar", baz: 42, scope: "should-be-dropped" },
+      ts: new Date("2024-01-01T00:00:00Z"),
+      stacktrace: "",
+    };
+
+    const flat = flattenMessage(msg);
+
+    expect(flat.foo).toBe("bar");
+    expect(flat.baz).toBe(42);
+    // Reserved key from the object message should be dropped
+    expect(flat.scope).toBe("my-scope");
+    expect(flat.level).toBe("INFO");
+  });
+
+  test("level is serialized as string name", () => {
+    const msg: LogMessage = {
+      scope: "test",
+      level: LogLevel.ERROR,
+      msg: "error msg",
+      ts: new Date(),
+      stacktrace: "",
+    };
+
+    const flat = flattenMessage(msg);
+    expect(flat.level).toBe("ERROR");
+  });
+});