Add integration test coverage for diagram feature

[Copilot]

The diagram feature (--diagram flag for get pi) lacked test coverage per the coding guidelines requirement that all implementations include tests.

Changes

• Integration tests in tests/integration/process-instances.test.ts:
  • PNG file output validation (verifies signature bytes, file creation)
  • Inline terminal display (validates iTerm2 protocol escape sequences)
  • Error handling for non-existent process instances

Testing Approach

Unit tests evaluated but not implemented — the diagram rendering depends on Puppeteer, headless Chrome, and API calls with tightly coupled private functions. Integration tests provide full end-to-end coverage.

// Example test pattern
test('get process instance with --diagram flag saves PNG to file', async () => {
  const result = await createProcessInstance({ processDefinitionId: 'simple-process' });
  const outputPath = join(tmpDir, 'diagram.png');
  
  execSync(`node src/index.ts get pi --key ${result.processInstanceKey} --diagram --output ${outputPath}`);
  
  const fileContent = readFileSync(outputPath);
  assert.strictEqual(fileContent[0], 0x89); // PNG signature validation
  // ...
});

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