sp_BlitzCache: Fix ai_prompt returning nulls when @AI > 0

[Copilot]

ai_prompt could be NULL when @AI = 1 or @AI = 2 because the prompt-building UPDATE filtered out rows where QueryPlan IS NULL, and the output SELECT would concatenate @AISystemPrompt + NULL → NULL.

Changes

• Remove QueryPlan IS NOT NULL gate from prompt builder — prompts are now built for all rows when @AI > 0
• Explicit missing-data messages in prompt text — ISNULL(..., N'N/A') replaced with CASE WHEN ... IS NULL THEN N'(Query text/plan could not be retrieved.)' for both QueryText and QueryPlan
• Both-null override — when both QueryText and QueryPlan are NULL, ai_prompt is set to a simple message with no metrics or system prompt:

  SET ai_prompt = N'Prompt not generated because we can''t find the query text or query plan.'

• AI cursor and @ai=2 advice — filter changed from AND QueryPlan IS NOT NULL to AND (QueryPlan IS NOT NULL OR QueryText IS NOT NULL) so API calls still fire when at least one artifact exists
• Output SELECT — both standard and expert mode [AI Prompt] columns now skip prepending @AISystemPrompt for the both-null fallback message
• Constitution cursor — expanded to include databases where only QueryText is available

Original prompt

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This section details on the original issue you should resolve

<issue_title>sp_BlitzCache: ai_advice or ai_prompt may be returning nulls</issue_title>
<issue_description>What is the current behavior?
During a very active and enjoyable live stream, we seemed to notice that sometimes we got nulls in the ai_advice, ai_prompt, ai_payload, etc columns of sp_BlitzCache when the @ai parameter was set to 1 or 2.

What is the expected behavior?

If @ai > 0, the ai_prompt column should never be null. It should always have a value based on the prompt and query metrics. If our prompt-building query can't get ONE of the two (query plan or query text), the prompt should say that those objects couldn't be retrieved. If our prompt-building query can't get EITHER of those (both query plan and query text are null), then the ai_prompt column should only say, "Prompt not generated because we can't find the query text or query plan." and not include any metrics or job description stuff.)</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes sp_BlitzCache: ai_advice or ai_prompt may be returning nulls #3855

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