⚡ perf: memoize hasSingleStore() in StoreManager

[lbajsarowicz]

Summary

• Memoize hasSingleStore() result in StoreManager to avoid redundant store collection loading on every call.
• The method previously called getStores(true) and counted results on every invocation. The code contained a TODO (MAGETWO-39902) acknowledging the need for caching.
• Cache is properly invalidated in reinitStores() and _resetState().

Performance Impact

Metric	Before	After
hasSingleStore()	Calls getStores(true) + count on every invocation	Single computation, cached in instance property
Cache invalidation	N/A	Reset on reinitStores() and _resetState()

Test Plan

• Unit test verifies memoization (repeated calls don't re-execute getList())
• Unit test verifies cache invalidation after reinitStores()
• Unit test verifies re-memoization after invalidation (two calls post-reinit, only one getList())
• PHPCS clean (requires CI validation)
• PHPStan clean (requires CI validation)

Ref: #40706

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[lbajsarowicz]

@magento run all tests