✅ test: protect synced LLM model references

[CodFrm]

Summary

• lock the account-sync entity contract for stable LLM Provider model references
• verify synced Backend bindings and structured Claude route targets preserve providerKey + modelKey
• prevent future server-side normalization from dropping stable model identity

This is the server-side compatibility guard for the paired Agentre desktop multi-model Provider change. Desktop implementation: agentre-hub/agentre#45.

Testing

• make test
  • Go race suite passed
  • frontend: 19 files / 256 tests passed

Review and verification

• Worktree clean at 14e88319e81780674a007f7677b92565a65acd57
• No outstanding findings, blocked tasks, or unobserved requirements for this server slice