⚡ perf: optimize hasLinks() and hasSamples() in Downloadable product type

[lbajsarowicz]

Summary

• Optimize hasLinks() and hasSamples() in Downloadable product type to avoid loading full link/sample collections just to check existence.
• Both methods now check for already-cached data on the product object first (getDownloadableLinks() / getDownloadableSamples()), returning immediately without any DB query.
• Only when cached data is unavailable (cold cache), falls back to lightweight getSize() query instead of full getLinks()/getSamples() which load all objects with titles, prices, and joins.

Why this approach (not just getSize)

Call site analysis shows hasLinks() / hasSamples() are often followed by template code that loads the full collection. The collection result is cached on the product data object. So:

• Warm cache (most common in templates): cached check returns instantly, zero DB queries
• Cold cache (standalone boolean checks in hasOptions(), isSalable(), canConfigure()): getSize() issues lightweight COUNT instead of full collection load
• Net effect: never worse than before, significantly better for standalone boolean checks

Performance Impact

Method	Before	After (warm cache)	After (cold cache)
hasLinks()	Full getLinks() load	Instant (cached check)	SELECT COUNT(*)
hasSamples()	Full getSamples() load	Instant (cached check)	SELECT COUNT(*)

Test Plan

• Unit tests for cached data path (both true/false cases)
• Unit tests for DB fallback path when cached data is null
• Tests cover both hasLinks and hasSamples
• PHPCS clean (requires CI)
• PHPStan clean (requires CI)

Ref: #40727

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[lbajsarowicz]

@magento run all tests