General: Introduce wp_get_branch_version() helper to extract major.minor version safely.

This introduces a new helper function `wp_get_branch_version()` for extracting the WordPress branch version (major.minor) from a version string.

Currently, several different approaches are used across core to determine the branch version, including:

- `(float)` casting of `get_bloginfo( 'version' )`
- `substr( $ver, 0, 3 )`
- `explode( '.' )` based parsing
- `preg_split()` in `class-core-upgrader.php`

Some of these approaches are fragile or incorrect:

- `(float)` casting can produce incorrect values due to floating-point precision issues.
- `substr( $ver, 0, 3 )` breaks for versions >= 10.
- Multiple inconsistent approaches make maintenance harder.

This patch introduces `wp_get_branch_version()` using a string-based approach:

```php
function wp_get_branch_version( $version = '' ) {
    if ( '' === $version ) {
        $version = wp_get_wp_version();
    }

    $parts = preg_split( '/[.-]/', $version, 3 );

    return $parts[0] . '.' . ( $parts[1] ?? '0' );
}

Author: tejas0306

src/wp-admin/admin-header.php

@@ -191,7 +191,7 @@
 	$admin_body_class .= ' taxonomy-' . $current_screen->taxonomy;
 }
 
-$admin_body_class .= ' branch-' . str_replace( array( '.', ',' ), '-', (float) get_bloginfo( 'version' ) );
+$admin_body_class .= ' branch-' . str_replace( '.', '-', preg_replace( '/\.0$/', '', wp_get_branch_version() ) );
 $admin_body_class .= ' version-' . str_replace( '.', '-', preg_replace( '/^([.0-9]+).*/', '$1', get_bloginfo( 'version' ) ) );
 $admin_body_class .= ' admin-color-' . sanitize_html_class( get_user_option( 'admin_color' ), 'modern' );
 $admin_body_class .= ' locale-' . sanitize_html_class( strtolower( str_replace( '_', '-', get_user_locale() ) ) );

src/wp-admin/includes/class-core-upgrader.php

@@ -279,8 +279,8 @@ public function upgrade( $current, $args = array() ) {
 	public static function should_update_to_version( $offered_ver ) {
 		require ABSPATH . WPINC . '/version.php'; // $wp_version; // x.y.z
 
-		$current_branch = implode( '.', array_slice( preg_split( '/[.-]/', $wp_version ), 0, 2 ) ); // x.y
-		$new_branch     = implode( '.', array_slice( preg_split( '/[.-]/', $offered_ver ), 0, 2 ) ); // x.y
+		$current_branch = wp_get_branch_version( $wp_version ); // x.y
+		$new_branch     = wp_get_branch_version( $offered_ver ); // x.y
 
 		$current_is_development_version = (bool) strpos( $wp_version, '-' );
 

src/wp-admin/includes/class-wp-site-health-auto-updates.php

@@ -352,7 +352,9 @@ public function test_all_files_writable() {
 		$dev       = ( str_contains( $wp_version, '-' ) );
 		// Get the last stable version's files and test against that.
 		if ( ! $checksums && $dev ) {
-			$checksums = get_core_checksums( (float) $wp_version - 0.1, 'en_US' );
+			$parts     = preg_split( '/[.-]/', $wp_version, 3 );
+			$prev_ver  = $parts[0] . '.' . max( 0, (int) ( $parts[1] ?? 0 ) - 1 );
+			$checksums = get_core_checksums( $prev_ver, 'en_US' );
 		}
 
 		// There aren't always checksums for development releases, so just skip the test if we still can't find any.

src/wp-admin/includes/plugin-install.php

@@ -113,7 +113,7 @@ function plugins_api( $action, $args = array() ) {
 	}
 
 	if ( ! isset( $args->wp_version ) ) {
-		$args->wp_version = substr( wp_get_wp_version(), 0, 3 ); // x.y
+		$args->wp_version = wp_get_branch_version(); // x.y
 	}
 
 	/**

src/wp-admin/includes/theme.php

@@ -504,7 +504,7 @@ function themes_api( $action, $args = array() ) {
 	}
 
 	if ( ! isset( $args->wp_version ) ) {
-		$args->wp_version = substr( wp_get_wp_version(), 0, 3 ); // x.y
+		$args->wp_version = wp_get_branch_version(); // x.y
 	}
 
 	/**

src/wp-includes/block-template-utils.php

@@ -1553,7 +1553,7 @@ function wp_generate_block_templates_export_file() {
 	$theme_json_raw = $tree->get_data();
 	// If a version is defined, add a schema.
 	if ( $theme_json_raw['version'] ) {
-		$theme_json_version = 'wp/' . substr( $wp_version, 0, 3 );
+		$theme_json_version = 'wp/' . wp_get_branch_version( $wp_version );
 		$schema             = array( '$schema' => 'https://schemas.wp.org/' . $theme_json_version . '/theme.json' );
 		$theme_json_raw     = array_merge( $schema, $theme_json_raw );
 	}

src/wp-includes/functions.php

@@ -8977,6 +8977,28 @@ function wp_get_wp_version() {
 	return $wp_version;
 }
 
+/**
+ * Returns the major.minor "branch" version for a given WordPress version string.
+ *
+ * Extracts the first two version components (major and minor) from a WordPress
+ * version string, stripping any patch level and pre-release suffix.
+ *
+ * @since 7.1.0
+ *
+ * @param string $version Optional. A WordPress version string. Defaults to the
+ *                        current WordPress version from wp_get_wp_version().
+ * @return string The branch version string in "major.minor" format (e.g. "6.9").
+ */
+function wp_get_branch_version( $version = '' ) {
+	if ( '' === $version ) {
+		$version = wp_get_wp_version();
+	}
+
+	$parts = preg_split( '/[.-]/', $version, 3 );
+
+	return $parts[0] . '.' . ( $parts[1] ?? '0' );
+}
+
 /**
  * Checks compatibility with the current WordPress version.
  *