⚡ perf: track import bunch size incrementally instead of full serialize

[lbajsarowicz]

Summary

• Replace O(n²) strlen(serialize($bunchRows)) per-row check in AbstractEntity::_saveValidatedBunches() with incremental size tracking that serializes only the new row and adds to a running total.
• The original code re-serialized the ENTIRE accumulated bunch array on every row iteration to check the size limit. For a 1000-row bunch, this means ~500K total serialization calls.
• Includes key serialization overhead in the estimate to prevent drift for non-first bunches with non-sequential array keys.

Performance Impact

Metric	Before	After
Serialization calls per bunch	O(n²) — full array per row	O(n) — one row per row
1000-row bunch	~500K serialize operations	1000 serialize operations
Size estimate accuracy	Exact	Within bytes (includes key overhead)

Test Plan

• Unit test verifies incremental approach produces identical bunch splits as legacy full-serialize
• Test covers multi-bunch scenario with non-sequential keys (after flush)
• PHPCS clean (requires CI)
• PHPStan clean (requires CI)

Ref: #40726

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[lbajsarowicz]

@magento run all tests