fix(ui): keep completion-mode models in the playground chat dropdown

[yuneng-berri]

What

PR #36130 added a KNOWN_MODEL_MODES guard to isModelCompatibleWithEndpoint so models with unrecognized modes (batch, rerank, ocr, etc.) don't show up under chat-style playground endpoints. mode: completion (legacy text-completion models) wasn't in the ModelMode enum, so it got caught by the same guard and disappeared from the playground dropdown entirely, including the chat endpoint it actually works with — 36 models on the default price/context list.

Fix

Add ModelMode.COMPLETION = "completion" and map it to EndpointType.CHAT, same as the other chat-compatible modes.

Testing

• Added a case to EndpointUtils.test.tsx asserting a completion-mode model is compatible with CHAT/RESPONSES and not with unrelated endpoints (e.g. SPEECH).
• vitest run on the playground + chat_ui suites: 282 passed.
• Lint clean on the two changed files.

[greptile-apps]

Greptile Summary

Adds completion as a recognized model mode and classifies it as chat-compatible so legacy completion models remain available in playground model selectors.

• Extends the model-mode enum and endpoint mapping.
• Adds compatibility assertions for Chat, Responses, and an unrelated endpoint.

Confidence Score: 5/5

The PR appears safe to merge, with no concrete blocking or non-blocking issue established in the changed code.

The change restores completion-mode models to the existing chat-compatible filtering path and adds focused regression coverage without altering request construction or backend routing.

Important Files Changed

Filename	Overview
ui/litellm-dashboard/src/components/chat_ui/mode_endpoint_mapping.tsx	Recognizes completion-mode models and maps them to the existing chat-compatible endpoint behavior; no actionable defect was established.
ui/litellm-dashboard/src/app/(dashboard)/playground/components/chat_ui/EndpointUtils.test.tsx	Adds focused coverage confirming completion-mode filtering for compatible and incompatible endpoint categories.

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