Improve bash detection and test reliability

Co-authored-by: swissspidy <841956+swissspidy@users.noreply.github.com>

Author: Copilot

features/shell.feature

@@ -77,8 +77,8 @@ Feature: WordPress REPL
       """
     And STDERR should be empty
 
-    # SHELL pointing to non-bash shell should be ignored and fall back to /bin/bash.
-    When I try `SHELL=/bin/zsh wp shell --basic < session`
+    # SHELL pointing to non-bash binary should be ignored and fall back to /bin/bash.
+    When I try `SHELL=/bin/sh wp shell --basic < session`
     Then STDOUT should contain:
       """
       bool(true)

src/WP_CLI/Shell/REPL.php

@@ -161,8 +161,9 @@ private static function is_bash_shell( $shell_path ) {
 		if ( ! is_file( $shell_path ) || ! is_readable( $shell_path ) ) {
 			return false;
 		}
-		// Check if the basename contains 'bash'.
-		return false !== strpos( basename( $shell_path ), 'bash' );
+		// Check if the basename is exactly 'bash' or starts with 'bash' followed by a version/variant.
+		$basename = basename( $shell_path );
+		return 'bash' === $basename || 0 === strpos( $basename, 'bash-' );
 	}
 
 	private function set_history_file() {