fix: eliminate model info flicker during settings save

[daniel-lxs]

Fixes #4430

Problem

When settings are saved, dynamic providers (OpenRouter, LiteLLM, DeepInfra, Roo, etc.) briefly show generic fallback model info while their model lists reload, causing a visual flicker.

Solution

Added a simple in-memory cache in ApiOptions component that:

• Stores the last known ModelInfo when available
• Uses cached info as fallback when selectedModelInfo temporarily becomes unavailable
• Automatically updates when fresh model info loads

Implementation

Just 3 lines of React state:

const [lastKnownModelInfo, setLastKnownModelInfo] = useState<ModelInfo | undefined>()

useEffect(() => {
  if (selectedModelInfo) {
    setLastKnownModelInfo(selectedModelInfo)
  }
}, [selectedModelInfo])

const displayModelInfo = selectedModelInfo || lastKnownModelInfo

No persistence, no cross-window coordination, no background revalidation - just a simple bridge to prevent flicker during settings save.

Testing

Manually tested settings save flow - no flicker observed.

---

Important

Adds in-memory cache in ApiOptions to prevent model info flicker during settings save by using cached data as fallback.

• Behavior:
  • Adds in-memory cache in ApiOptions to store last known ModelInfo.
  • Uses cached ModelInfo as fallback when selectedModelInfo is unavailable.
  • Updates cache when new ModelInfo is loaded.
• Implementation:
  • Introduces lastKnownModelInfo state and useEffect to update it in ApiOptions.
  • Replaces selectedModelInfo with displayModelInfo in relevant JSX conditions.
• Testing:
  • Manually tested settings save flow to confirm flicker is resolved.

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[roomote]

Rooviewer   See task on Roo Cloud

Review found 2 areas where the flicker fix is incomplete:

• Line 783: SimpleThinkingBudget for "roo" provider still uses selectedModelInfo instead of displayModelInfo, causing flicker during settings save
• Line 754: Deprecated model error check uses selectedModelInfo?.deprecated instead of displayModelInfo?.deprecated, causing the error message to flicker

_{Mention @roomote in a comment to request specific changes to this pull request or fix all unresolved issues.}

[daniel-lxs]

Closing this PR as it doesn't address the root cause of #4430. The issue is not about model info flicker in the UI, but about the backend unconditionally rebuilding the API handler on Save, which triggers context condensing. Opening a new PR with the correct fix.