Rename _wcpay_feature_tokenized_cart_prb Feature Flag to _wcpay_feature_tokenized_cart_ece

[frosso]

To unify the tokenized cart approach, we’ll rename the _wcpay_feature_tokenized_cart_prb feature flag to _wcpay_feature_tokenized_cart_ece. This aligns with the new structure in which the ECE and Tokenized Cart implementations will share common ground, with the Store API as the point of interface.

For background: pbIwZe-2ur-p2

Acceptance Criteria:

• _wcpay_feature_tokenized_cart_prb flag is renamed to _wcpay_feature_tokenized_cart_ece throughout the codebase.
• Verify no functionality is impacted by the renaming.
• Confirm that tests relying on the original feature flag are updated accordingly.
• All related documentation, comments, and references are updated to reflect the new flag name