fix: preserve trailing slash in MCP server URLs (#21212)

Stop trimming trailing slashes from MCP server URLs on save. Some MCP
servers (e.g. Bitrix24) require a trailing slash; removing it triggers a
301 redirect that drops Authorization headers, resulting in 400 errors.

The trailing-slash trim is now skipped when the connection type is 'mcp',
while OpenAPI connections continue to have it stripped as before.

Fixes open-webui#21179

src/lib/components/AddToolServerModal.svelte

@@ -250,8 +250,12 @@
 	const submitHandler = async () => {
 		loading = true;
 
-		// remove trailing slash from url
-		url = url.replace(/\/$/, '');
+		// remove trailing slash from url for non-MCP connections
+		// MCP servers may require a trailing slash; stripping it can cause
+		// 301 redirects that lose auth headers (see #21179)
+		if (type !== 'mcp') {
+			url = url.replace(/\/$/, '');
+		}
 		if (id.includes(':') || id.includes('|')) {
 			toast.error($i18n.t('ID cannot contain ":" or "|" characters'));
 			loading = false;