fix(playground): create agent in getActionSpace when using agentFactory mode

[quanru]

Summary

• Fix structured parameter forms not displaying in Chrome Extension playground
• When using agentFactory mode, create agent in getActionSpace if it doesn't exist

Problem

After commit 89d7f6c7, when using agentFactory mode in LocalExecutionAdapter:

• The agent was only created during executeAction
• But getActionSpace was called earlier during page initialization
• This caused getActionSpace to return an empty array []
• As a result, needsStructuredParams was always false
• All playground methods displayed as plain TextArea instead of structured parameter forms (e.g., direction dropdown for aiScroll)

Solution

In getActionSpace, if agent is null but agentFactory is available:

1. Create the agent using the factory
2. Get the actionSpace from the created agent
3. Keep the agent for later use instead of destroying it

Test plan

• Open Chrome Extension playground
• Select aiScroll method - should show direction dropdown and distance input instead of plain TextArea
• Select aiInput method - should show structured form with value and locate fields
• Verify other methods like aiTap, aiHover also show proper structured forms

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