Add index on catalog_product_entity.type_id (#37431)

[lbajsarowicz]

Description (*)

catalog_product_entity.type_id has no index. Core filters on it in the configurable and downloadable export row customizers, the admin widget product chooser, the low-stock report collection and the product frontend action synchronizer, and every one of those is a full table scan.

This adds a btree index on type_id and whitelists it. Declarative schema has no prefix-length support (index.xsd only knows a column name), so this is a full-column index.

Measured on a copy of the table grown to 307,200 rows (4,096 configurable, 303,104 simple), MySQL 8.0, medians of 3 from SHOW PROFILES:

Query	before	after	plan before	plan after
COUNT(*) WHERE type_id='configurable'	13.9 ms	0.42 ms	ALL, 304,569 rows	ref, 4,096 rows, Using index
entity_id, sku WHERE type_id='configurable' ORDER BY entity_id LIMIT 200	0.99 ms	0.19 ms	index PRIMARY	ref, Using index condition
COUNT(*) WHERE type_id='simple'	15.6 ms	20.7 ms	ALL	ref, 305,991 rows, Using index

The last row is the trade-off: for the dominant value the optimizer prefers the 130-byte key over the clustered scan and gets slower. Core never filters on type_id='simple' alone, and a prefix index that would shrink that cost is not expressible in declarative schema.

Line wrapping elsewhere in db_schema.xml is there because the Static Tests build fails on pre-existing line-length warnings in any touched file.

Local gates: unit and static pass. In the Catalog integration suite, SortingTest::testProductListOutOfStockSortOrderWithMysql fails identically on a clean 2.4-develop checkout in the same environment, so it is not related to this change.

Fixed Issues (if relevant)

1. Fixes [Perf] Add index on type_id column for catalog_product_entity table #37431

Manual testing scenarios (*)

1. bin/magento setup:upgrade on an existing install, then SHOW INDEX FROM catalog_product_entity shows CATALOG_PRODUCT_ENTITY_TYPE_ID.
2. bin/magento setup:db-declaration:generate-whitelist --module-name=Magento_Catalog produces no diff.
3. EXPLAIN SELECT COUNT(*) FROM catalog_product_entity WHERE type_id='configurable' uses the new key.

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• README.md files for modified modules are updated and included in the pull request if any README.md predefined sections require an update
• All automated tests passed successfully (all builds are green)

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[lbajsarowicz]

@magento run all tests