Fix 8 spec issues found during SDK testing

1. Streaming RPCs: content-type → application/connect+json, add
   Connect-Protocol-Version and Connect-Timeout-Ms headers
2. EndEvent.exitCode: marked deprecated, document status string parsing
3. envdAccessToken: clarify only returned with secure: true, mark nullable
4. LogLevel: fix description ("State of the sandbox" → log severity)
5. Sandbox.domain: mark deprecated (always null)
6. GET /templates/{id}/files/{hash}: change 201 → 200 response

Issues 4 (pagination response) and 5 (DELETE with body) are upstream
API design decisions that cannot be fixed in the spec alone.

Author: beran-t