test: add eval for create chart tool call in analyst agent

[djbarnwal]

Adds eval for create_chart tool call

Add TestAnalystCharts with 4 subtests covering the create_chart tool in the analyst agent

https://linear.app/rilldata/issue/APP-725/ai-improve-analyst-agents-visualization-logic

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!