Mock Hcm.prompt

# Phase 4 — Standalone Mock HCM server

## Context to read before starting
- `.cursorrules`
- `context/05-hcm-contract.md`
- `context/06-testing.md` — "Mock HCM server" section

Phases 1, 2, and 3 are complete.

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## Task

Refine and complete the HCM mock server so it works as a deployable standalone HTTP server, with in-memory state and basic logic that simulates real HCM behavior.

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## Deliverables checklist

### 1. `mock-hcm.service.ts`
- [ ] In-memory state: map of processed requests by `idempotencyKey`
- [ ] Attempt counter by `idempotencyKey` (to simulate successful retry on 2nd attempt)
- [ ] Method `resetState()` — clears all state
- [ ] Method `setScenario(scenario)` — changes active scenario
- [ ] Method `getScenario()` — returns current scenario
- [ ] Scenario `success_on_retry_2`: fails on first attempt, returns 201 on second

### 2. mock-hcm.controller.ts

`GET /hcm/balance`:
| X-Mock-Scenario | Behavior |
|---|---|
| `success` (default) | 200 with hardcoded balance: `availableDays: 10`, `totalAssigned: 15` |
| `not_found` | 404 |
| `timeout` | `await sleep(5000)` — forces microservice timeout |
| `server_error` | 503 |

`POST /hcm/time-off`:
| X-Mock-Scenario | Behavior |
|---|---|
| `success` (default) | 201 with generated `hcmRequestId: uuid()` |
| `insufficient_balance` | 422 with `{ error: "INSUFFICIENT_BALANCE", availableDays: 1.5 }` |
| `invalid_dimensions` | 400 |
| `server_error` | 503 |
| `idempotent` | If `idempotencyKey` exists: 200 with `idempotent: true`. Otherwise: 201 |
| `success_on_retry_2` | First attempt: 503. Second attempt with same key: 201 |

`POST /mock/control` (internal control endpoint):
- Body: `{ scenario: string }` — changes active scenario
- Body: `{ action: "reset" }` — clears state

### 3. mock-hcm.helpers.ts