Fix nil-pointer panic in ai resolver when time_range/comparison_time_range are omitted

[nishantmonu51]

Fixes a guaranteed nil-pointer panic in the ai resolver when time_range or comparison_time_range is omitted from the resolver properties.

• ResolveInteractive unconditionally dereferences r.props.TimeRange and r.props.ComparisonTimeRange when building AnalystAgentArgs, but both are optional *metricsview.TimeRange fields left nil when absent. Since comparison_time_range is almost never set, essentially every AI report without it panicked; the panic was recovered by the controller, so the report perpetually failed with a reconcile error instead of crashing the process.
• Fix: default both pointers to &metricsview.TimeRange{} in newAI. An empty time range is already a no-op in resolveTimeRange (its IsZero() guard), so the agent args now receive zero time.Time values, meaning "no time bounds".

Steps to reproduce (before this fix):

1. In a Rill project with AI enabled, create a report whose data section uses the AI resolver with only a prompt, e.g. data: { ai: { agent: analyst_agent, prompt: "Summarize sales", is_report: true } } — do not set time_range or comparison_time_range.
2. Trigger the report (via its schedule or "Run now").
3. The report fails every time: the resource shows a reconcile error and the runtime logs contain a recovered nil-pointer panic in aiResolver.ResolveInteractive. Setting only time_range still fails on the comparison_time_range dereference.

To test: repeat the steps above on this branch; the report executes and produces an AI summary. Also verify that a report with an explicit time_range expression (e.g. expression: P7D) still resolves the range correctly.

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!

[nishantmonu51]

@codex: review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Chef's kiss.

Reviewed commit: 00d55dd200

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