Disable AI search when literal search finds no matches

[Mbeaulne]

Description

AI search is now disabled when a literal (lexical) search returns no matches. Previously, when no lexical hits were found, AI search would fall back to a sampled "browse pool" built by evenly sampling the full component index, allowing the AI reranker to run against a representative spread of components. This behavior has been removed — if the query doesn't match anything lexically, the AI search button is disabled and the reranker is not called.

This removes the sampleEvenly utility function and the indexEntryToLexicalMatch fallback path from the Dashboard view, and updates buildAiCandidateMatches to return an empty array immediately when lexical search finds nothing.

Related Issue and Pull requests

Type of Change

• Bug fix
• New feature
• Improvement
• Cleanup/Refactor
• Breaking change
• Documentation update

Checklist

• I have tested this does not break current pipelines / runs functionality
• I have tested the changes on staging

Screenshots (if applicable)

Test Instructions

1. Open the Dashboard components view with AI search configured.
2. Enter a query that has no lexical matches (e.g., a nonsense string like qqzznomatch).
3. Confirm the "AI search" button is disabled and clicking it does not trigger a rerank call.
4. Enter a query that does have lexical matches and confirm AI search remains functional.

Additional Comments

The previous browse-pool fallback was intended to give the AI model a representative sample when literal search found nothing. This PR removes that behavior, meaning AI search is only available as a refinement step when there are already some lexical candidates to rerank.

[github-actions]

🎩 Preview

A preview build has been created at: 06-22-fix-ai-search-no-results-fallback/83db60e

[Mbeaulne]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Simplify component search result cards #2459
• Add context-aware component search suggestions #2458
• Add component discovery docs #2457
• Add component search analytics baseline #2456
• Limit dashboard component browse rendering #2454
• Add component search empty state suggestions #2453
• Decouple editor component search input #2452
• Polish component search explanations #2451
• Add compatible component suggestions #2450
• Add component collection search results #2449
• Add component lifecycle badges #2448
• Add component search URL state #2447
• Disable AI search when literal search finds no matches #2444 👈 (View in Graphite)
• Debounce component search input #2433
• add client-side embeddings cached in IndexedDB #2432
• Add search quality expectation tests for lexical search #2431
• Add deep AI search to rerank all components in selected sources #2430
• Add source-diverse AI rerank candidate pool #2429
• Add negative constraint parsing to lexical search #2428
• Add fuzzy/typo-tolerant matching for component name and I/O fields #2427
• Improve component search scoring relevance #2426
• Add synonym expansion to component lexical search #2425
• Normalize component search tokens for better matching #2424
• Expand component search indexing fields #2423
• master

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