fix(workbuddy): expose deepseek-v4.1-flash via catalog alias

[Circumsized]

What

Some WorkBuddy Global accounts do not expose deepseek-v4.1-flash in the upstream catalog even though the client UI offers it. This change removes the overly strict CLI-agent-only model filtering and adds a small compatibility alias so deepseek-v4.1-flash remains routable through the gateway.

Why

• The previous isCLIAgent filter only allowed models from cli/codebuddy/workbuddy agents.
• In practice, WorkBuddy Global accounts may omit deepseek-v4.1-flash from the catalog response while the client UI still supports it.
• This led to a visible mismatch: the model was usable in WorkBuddy AI but missing from /v1/models in cli2api.

How

• Remove isCLIAgent-based model filtering in internal/providers/workbuddy/client.go so all non-disabled upstream catalog models are exposed.
• Add appendAliasModels with a compatibility alias mapping deepseek-v4.1-flash to the existing deep-model capabilities when the upstream catalog does not contain it.
• Update the affected test to reflect the new behavior.
• Remove the now-unused isCLIAgent helper.

Verification

• Rebuilt the container from this branch.
• Confirmed /v1/models now includes deepseek-v4.1-flash.
• Confirmed the service health check remains healthy.