Add negative/error test cases

[jtdub]

Problem

There are no tests for error paths:

• Invalid or unknown platform names
• Malformed configuration text
• Missing required fields in requests
• 404 responses for non-existent remediation/report/job IDs
• Invalid regex patterns in search
• Empty config lists for merge
• Requesting results for an incomplete batch job

The error handling code in routers and services is completely untested.

Suggested Fix

Add test cases covering at least:

def test_compare_configs_invalid_platform(client):
    response = client.post("/api/v1/configs/compare", json={...})
    # assert appropriate status code

def test_get_report_not_found(client):
    response = client.get("/api/v1/reports/nonexistent-id/summary")
    assert response.status_code == 404

def test_search_invalid_regex(client):
    response = client.post("/api/v1/configs/search", json={
        "match_rules": {"regex": "[invalid"}
    })
    assert response.status_code == 400