Add test coverage for message size validation

[Copilot]

Adds missing test coverage for messageSizeWithinLimit() utility function and _sendMessage() wrapper introduced in #168.

Test coverage added:

• messageSizeWithinLimit() validation for messages within/exceeding limits, complex objects, and boundary conditions
• _sendMessage() error handling: MessageTooLargeError thrown, onError callback invoked with correct message
• Edge cases: exact size limit, one byte over

Supporting changes:

• Jest setup file to polyfill TextEncoder for jsdom environment
• Helper functions to reduce test duplication
• Named constants for test data sizes

Example test structure:

test("should throw MessageTooLargeError for oversized messages", async () => {
  await client.connect();
  const largeData = createOversizedData();
  expect(() => {
    client.sendClientMessage("test", { data: largeData });
  }).toThrow(MessageTooLargeError);
});

All 15 tests passing. No security issues detected.

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