Fix customer email length validation before save (#40842)

[ajith107]

Summary

• Add Magento\Customer\Model\Validator\Email to validate customer email length against the 255-character database limit (customer_entity.email).
• Register the validator in the customer save validation group so validation runs for storefront registration, admin, API, and other save paths.
• Return a user-friendly "Email" uses too many characters. message instead of a raw NoSuchEntityException that exposes websiteId.

Fixes #40842

Issue

When registering with an email longer than 255 characters, Magento attempted to save to the database and then failed with:

No such entity with email = ..., websiteId = 1

Manual testing

• Storefront /customer/account/create with 256+ character email shows "Email" uses too many characters.
• Storefront registration with a valid email succeeds

Automated testing

php vendor/bin/phpunit app/code/Magento/Customer/Test/Unit/Model/Validator/EmailTest.php

Test plan

• Register a customer with an email of 256+ characters on the storefront; confirm the validation message is shown.
• Register a customer with a valid email; confirm account creation succeeds.
• (Optional) Create a customer in Admin with a 256+ character email; confirm the same validation message.

[m2-assistant]

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[ajith107]

@magento run all tests

[ajith107]

@magento run all tests