[Studio] Normalize AI draft model

[Aias00]

Summary

• trim the AI chat draft model from navigation state before applying it
• drop empty model values after normalization so the chat page keeps its default selection
• add regression coverage for both cases

Verification

• npm --prefix web test -- --run src/pages/ai/chatDraft.test.ts
• (cd web && npm exec eslint -- src/pages/ai/chatDraft.ts src/pages/ai/chatDraft.test.ts)
• npm --prefix web run build

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[Aias00]

Superseded by #642. Folded into the consolidated AI resource tools PR so AI draft normalization and tool-facing AI resource behavior are reviewed together.