TAN-7573: Fix missing Auto insights for surveys with many pages

[EdwinKato]

Changelog

Fixed

• On the AI analysis view for a survey or project, the Auto insights carousel no longer silently disappears when the survey is split across many pages. Previously, multi-page surveys with more than 50 total fields (including page/section dividers) would produce no heatmap cells and the Auto insights area would render as blank. Now layout-only fields (pages) are not counted against the 50-field cap, so these surveys get Auto insights as expected.
• When an analysis genuinely has more than 50 questions, the Auto insights area now shows an explanatory message stating the feature isn't available for analyses with that many questions, instead of rendering a blank space with no feedback.

Technical

• Added Analysis::Analysis#submission_custom_fields reusing CustomField#supports_submission? (the same predicate used elsewhere in the analysis serializer) to distinguish real questions from layout-only fields.
• Exposed a new boolean attribute auto_insights_too_many_fields on the Analysis serializer so the frontend can render the appropriate empty state.
• The fix does not retroactively regenerate heatmap cells for existing analyses that were affected — HeatmapGenerationJob only fires on analysis create/update. For the affected Wokingham tenant, Support can either re-save the analysis or run Analysis::HeatmapGenerationJob.new.run(analysis) from a Rails console to regenerate.

For translators

• New string app.containers.AdminPage.projects.project.analysis.heatmap.autoInsightsTooManyFields (Auto-insights are not available because this analysis includes more than 50 questions. With that many questions, automated correlations become unreliable and are not shown.) — shown in the Auto insights area of the admin AI analysis view when the analysis contains more than 50 questions, explaining why the feature is unavailable.

[notion-workspace]

🐛 Auto insights: not working → button does not appear

[cl-dev-bot]

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📖	Changelog provided 🎉
📖	Notion issue: TAN-7573
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