fix: use distinct scheme_name on APIKeyHeader so Swagger shows correct curl headers (#80)

[Som23Git]

Brief updates:

• Add scheme_name="AdminApiKey" to API_KEY_HEADER (X-Admin-API-Key)
• Add scheme_name="UserApiKey" to USER_API_KEY_HEADER (X-API-Key)
• FastAPI collapses both into one scheme when scheme_name is missing, causing Swagger to show X-API-Key for all routes including admin ones
• Add regression test (test_openapi_schema.py) that validates both schemes exist in /openapi.json with correct header names. It's additional but, it's good way to test it programmatically.

Manual tests:

Addresses this comment.

# Request
curl -X 'POST' \
  'http://localhost:8057/admin/users' \
  -H 'accept: application/json' \
  -H 'X-Admin-API-Key: newadmintoken' \
  -H 'Content-Type: application/json' \
  -d '{
  "email": "user@example.com",
  "name": "string",
  "image_url": "string",
  "max_concurrent_bots": 0,
  "data": {
    "additionalProp1": {}
  }
}'

# Response
{
  "email": "user@example.com",
  "name": "string",
  "image_url": "string",
  "max_concurrent_bots": 0,
  "data": {},
  "id": 3,
  "created_at": "2026-05-05T20:30:37.591537"
}

Closes #80

And, attaching my virtually consented/signed Individual CLA per CONTRIBUTING.md

individual-cla.pdf