PUT /config/router repeatedly prefixes canonical backend_ref names #2729

Summary

PUT /config/router is not idempotent when it receives the canonical document returned by GET /config/router.

The Dashboard configuration path does not show this behavior: it reads and writes CanonicalConfig directly through /api/router/config/all and /api/router/config/update. The management API instead parses the submitted canonical document and exports it again through CanonicalConfigFromRouterConfig, which prefixes canonical backend reference names a second time.

Reproduction

Start with a valid v0.3 provider model whose canonical backend reference is:

providers:
  models:
    - name: test-model
      backend_refs:
        - name: test-model_primary
          endpoint: localhost:8000
          protocol: http

1. Read the active document with GET /config/router.
2. Submit that response unchanged with PUT /config/router.
3. Read the document again.

Actual result

The backend reference name becomes:

name: test-model_test-model_primary

Repeating the same round trip adds another prefix.

Expected result

Submitting the canonical document returned by GET /config/router should not change it.

Root cause

canonicalEndpointName always returns modelName + "_" + backendRef.Name, even when backendRef.Name already starts with the generated model prefix. The management update path invokes CanonicalConfigFromRouterConfig after parsing the submitted document.

The latent prefix behavior was introduced by #1553, and the management update normalization path was introduced by #1660. The issue is present in v0.3.0 and current main.

Impact

• External control planes cannot safely perform read-modify-write updates.
• Configuration drift checks report false mismatches.
• Rollback code that republishes a previously read document can mutate it again.
• Repeated retries can keep extending backend reference names.

Proposed fix

Make canonical endpoint-name generation idempotent when the backend reference already contains the model prefix, and add regression coverage for:

• canonical export of an already-canonical backend reference;
• an unchanged GET /config/router → PUT /config/router round trip;
• repeated round trips remaining stable.