Enrich API specs with real response codes from codebase

Audited all 53 endpoints against Go handler implementations in the infra
repo to extract actual HTTP status codes, error messages, and response
types. Added missing error response codes:

REST API:
- POST /sandboxes: added 403, 404, 429
- GET/DELETE /sandboxes/{id}: added 403
- POST pause: added 400, 403
- POST resume: added 400, 403, 429
- POST connect: added 403, 429
- POST refreshes: added 400, 500
- GET logs v1/v2: added 400
- GET team metrics: added 403
- Template CRUD: added 403, 404, 409, 429, 503 per endpoint

Data Plane:
- POST /init: added 400, 401
- All filesystem/process Connect RPC endpoints: added 400, 401, 404, 500

Also added 429, 503, 507 to standard response definitions.
Coverage: 53/53 endpoints now have error responses (was 36/53).

Author: beran-t