Clarify expected behavior for invalid weekday values in WP_Locale::get_weekday()

[shishir1]

While working on a pull request to prevent an undefined index notice in WP_Locale::get_weekday(), it became clear that the current behavior for invalid weekday values (e.g. 7) is intentional and covered by existing unit tests.

This ticket is to discuss whether the current behavior should remain as-is, or whether a more explicit developer-facing warning (such as using _doing_it_wrong()) would be more appropriate, particularly for newer PHP versions.

Related PR: ​https://github.com/WordPress/wordpress-develop/pull/10722

Feedback and guidance would be appreciated before proposing any changes.

[westonruter]

This is related to #34688 to follow up on code introduced in [36292].

[swissspidy]

At first glance I don't see why we would want to replace perfectly valid error flow (PHP notice/warning) with our own validatiob & doing_it_wrong.

The current behavior already tells the developer that their code is wrong.

[westonruter]

With PHPStan ($61175), static analysis would catch this via:

src/wp-includes/class-wp-locale.php

@@ -262 +262 @@
          *
          * @since 2.1.0
          *
-         * @param int $weekday_number 0 for Sunday through 6 Saturday.
+         * @param int<0, 6> $weekday_number 0 for Sunday through 6 Saturday.
          * @return string Full translated weekday.
          */
         public function get_weekday( $weekday_number ) {

Also, the issue with an invalid argument being passed to ::get_weekday() is also present for ::get_weekday_initial(), ::get_weekday_abbrev(), etc.