This PR is a cherry-pick of #65965 to release/10.9.

Original PR Description

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• I have followed the WooCommerce Contributing Guidelines and the WordPress Coding Standards.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• I have reviewed my code for security best practices.
• Following the above guidelines will result in quick merges and clear and detailed feedback when appropriate.

Changes proposed in this Pull Request:

PR #64394 added get_entry_count(): int as a required method on FeedInterface (shipped in 10.9.0). Because that interface is implemented by third-party code — notably the WooCommerce Stripe Gateway's WC_Stripe_Agentic_Commerce_Csv_Feed — adding a required method is a backward-incompatible change. Stripe Gateway versions that predate their own get_entry_count() implementation (added in Stripe 10.7) fatal on load under 10.9.0:

PHP Fatal error: Class WC_Stripe_Agentic_Commerce_Csv_Feed contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (Automattic\WooCommerce\Internal\ProductFeed\Feed\FeedInterface::get_entry_count) in .../woocommerce-gateway-stripe/includes/agentic-commerce/class-wc-stripe-agentic-commerce-csv-feed.php on line 31

This forced a revert of the 10.9.0 tag on WP Cloud. No core code calls get_entry_count() through the interface — it is only used on the concrete JsonFileFeed, and consumers (like Stripe) call it on their own feed classes. This PR removes the method from the interface to restore compatibility, keeping the implementation and its tests on JsonFileFeed intact.

Bug introduced in PR #64394.

How to test the changes in this Pull Request:

1. Check out a copy of the WooCommerce Stripe Gateway at a version before its own get_entry_count() was added (Stripe < 10.7), so its WC_Stripe_Agentic_Commerce_Csv_Feed implements FeedInterface without get_entry_count().
2. On trunk before this change, activate both WooCommerce and that Stripe Gateway version (or require the feed class) and confirm the fatal error above is raised.
3. Check out this branch and repeat — the class loads without a fatal error.
4. Confirm core feed behaviour is unchanged: pnpm --filter=@woocommerce/plugin-woocommerce test:php:env -- --filter 'JsonFileFeedTest|ProductWalkerTest' passes (19 tests / 453 assertions), including JsonFileFeed::get_entry_count() coverage.

Testing that has already taken place:

• pnpm --filter=@woocommerce/plugin-woocommerce test:php:env -- --filter 'JsonFileFeedTest|ProductWalkerTest' — 19 tests, 453 assertions, all passing.
• pnpm --filter=@woocommerce/plugin-woocommerce lint:php:changes — clean.
• composer exec -- phpstan analyse .../FeedInterface.php .../JsonFileFeed.php --memory-limit=2G — no errors.

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