use codehost_name to resolve codehost when loading service via openapi

[Cynthia-0203]

What this PR does / Why we need it:

The OpenAPI "create K8s YAML service from codebase" endpoint previously identified the code host via the codehostId path parameter. This PR changes it to accept codehost_name instead, and the backend resolves the code host by its alias (system-level) to obtain the codehost_id before loading the service.

What is changed and how it works?

• request field CodehostID int → CodehostName string; Validate() now requires codehost_name to be non-empty.
• route changed from /load/:codehostId to /load.

Does this PR introduce a user-facing change?

• API change
• database schema change
• upgrade assistant change
• change in non-functional attributes such as efficiency or availability
• fix of a previous issue

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This change is