Add chaos behavior for probabilistic fault injection

[tugkanboz]

Closes #918.

Adds a chaos behavior to the existing _behaviors framework. It probabilistically injects errors (replacing the response with an error status code) and/or random latency. Both are independent and all fields default to 0, so the behavior is a no-op unless explicitly configured. No new dependencies, just Math.random.

Example:

{
  "responses": [{ "is": { "statusCode": 200, "body": "ok" } }],
  "_behaviors": {
    "chaos": {
      "errorRate": 0.1,
      "errorStatusCode": 503,
      "latencyRate": 0.05,
      "maxLatencyMs": 1000
    }
  }
}

Changes:

• behavior + validation schema in src/models/behaviors.js
• 12 unit tests in test/models/behaviors/chaosTest.js
• doc page src/views/docs/api/behaviors/chaos.ejs plus entry in behaviors.ejs

All 750 existing unit tests still pass and lint is clean.

Naming question from #918 is still open: chaos, fault, or faults? Happy to rename before merge.

[sonarqubecloud]

Quality Gate failed

Failed conditions
3 Security Hotspots

See analysis details on SonarQube Cloud

[tugkanboz]

On the 3 SonarCloud security hotspots: they all flag Math.random() in the new chaos behavior. These are intentional. Chaos fault injection only needs uniform probability, not cryptographic randomness, and switching to crypto.randomInt would add cost without behavioral value. The same Math.random pattern already appears in the documented example for the wait behavior. Happy to take a different approach if you'd prefer.