feat: AI nodes usability fixes + Summarization Chain V2 (#7949)

Fixes:
- Refactor connection snapping when dragging and enable it also for
non-main connection types
- Fix propagation of errors from sub-nodes
- Fix chat scrolling when sending/receiving messages
- Prevent empty chat messages
- Fix sub-node selected styles
- Fix output names text overflow

Usability improvements:
- Auto-add manual chat trigger for agents & chain nodes
- Various labels and description updates
- Make the output parser input optional for Basic LLM Chain
- Summarization Chain V2 with a simplified document loader & text
chunking mode

#### How to test the change:
Example workflow showcasing different operation mode of the new
summarization chain:

[Summarization_V2.json](https://github.com/n8n-io/n8n/files/13599901/Summarization_V2.json)


## Issues fixed
Include links to Github issue or Community forum post or **Linear
ticket**:
> Important in order to close automatically and provide context to
reviewers
-
https://www.notion.so/n8n/David-Langchain-Posthog-notes-7a9294938420403095f4508f1a21d31d
- https://linear.app/n8n/issue/N8N-7070/ux-fixes-batch
- https://linear.app/n8n/issue/N8N-7071/ai-sub-node-bugs


## Review / Merge checklist
- [x] PR title and summary are descriptive. **Remember, the title
automatically goes into the changelog. Use `(no-changelog)` otherwise.**
([conventions](https://github.com/n8n-io/n8n/blob/master/.github/pull_request_title_conventions.md))
- [x] [Docs updated](https://github.com/n8n-io/n8n-docs) or follow-up
ticket created.
- [ ] Tests included.
> A bug is not considered fixed, unless a test is added to prevent it
from happening again. A feature is not complete without tests.
  >
> *(internal)* You can use Slack commands to trigger [e2e
tests](https://www.notion.so/n8n/How-to-use-Test-Instances-d65f49dfc51f441ea44367fb6f67eb0a?pvs=4#a39f9e5ba64a48b58a71d81c837e8227)
or [deploy test
instance](https://www.notion.so/n8n/How-to-use-Test-Instances-d65f49dfc51f441ea44367fb6f67eb0a?pvs=4#f6a177d32bde4b57ae2da0b8e454bfce)
or [deploy early access version on
Cloud](https://www.notion.so/n8n/Cloudbot-3dbe779836004972b7057bc989526998?pvs=4#fef2d36ab02247e1a0f65a74f6fb534e).

---------

Signed-off-by: Oleg Ivaniv <me@olegivaniv.com>
Co-authored-by: Elias Meire <elias@meire.dev>

packages/@n8n/nodes-langchain/nodes/chains/ChainLLM/ChainLlm.node.ts

@@ -191,13 +191,33 @@ async function getChain(
 	return Array.isArray(response) ? response : [response];
 }
 
+function getInputs(parameters: IDataObject) {
+	const hasOutputParser = parameters?.hasOutputParser;
+	const inputs = [
+		{ displayName: '', type: NodeConnectionType.Main },
+		{
+			displayName: 'Model',
+			maxConnections: 1,
+			type: NodeConnectionType.AiLanguageModel,
+			required: true,
+		},
+	];
+
+	// If `hasOutputParser` is undefined it must be version 1.1 or earlier so we
+	// always add the output parser input
+	if (hasOutputParser === undefined || hasOutputParser === true) {
+		inputs.push({ displayName: 'Output Parser', type: NodeConnectionType.AiOutputParser });
+	}
+	return inputs;
+}
+
 export class ChainLlm implements INodeType {
 	description: INodeTypeDescription = {
 		displayName: 'Basic LLM Chain',
 		name: 'chainLlm',
 		icon: 'fa:link',
 		group: ['transform'],
-		version: [1, 1.1],
+		version: [1, 1.1, 1.2],
 		description: 'A simple chain to prompt a large language model',
 		defaults: {
 			name: 'Basic LLM Chain',
@@ -217,25 +237,11 @@ export class ChainLlm implements INodeType {
 				],
 			},
 		},
-		// eslint-disable-next-line n8n-nodes-base/node-class-description-inputs-wrong-regular-node
-		inputs: [
-			NodeConnectionType.Main,
-			{
-				displayName: 'Model',
-				maxConnections: 1,
-				type: NodeConnectionType.AiLanguageModel,
-				required: true,
-			},
-			{
-				displayName: 'Output Parser',
-				type: NodeConnectionType.AiOutputParser,
-				required: false,
-			},
-		],
+		inputs: `={{ ((parameter) => { ${getInputs.toString()}; return getInputs(parameter) })($parameter) }}`,
 		outputs: [NodeConnectionType.Main],
 		credentials: [],
 		properties: [
-			getTemplateNoticeField(1951),
+			getTemplateNoticeField(1978),
 			{
 				displayName: 'Prompt',
 				name: 'prompt',
@@ -256,7 +262,7 @@ export class ChainLlm implements INodeType {
 				default: '={{ $json.chat_input }}',
 				displayOptions: {
 					show: {
-						'@version': [1.1],
+						'@version': [1.1, 1.2],
 					},
 				},
 			},
@@ -400,6 +406,28 @@ export class ChainLlm implements INodeType {
 					},
 				],
 			},
+			{
+				displayName: 'Require Specific Output Format',
+				name: 'hasOutputParser',
+				type: 'boolean',
+				default: false,
+				displayOptions: {
+					show: {
+						'@version': [1.2],
+					},
+				},
+			},
+			{
+				displayName: `Connect an <a data-action='openSelectiveNodeCreator' data-action-parameter-connectiontype='${NodeConnectionType.AiOutputParser}'>output parser</a> on the canvas to specify the output format you require`,
+				name: 'notice',
+				type: 'notice',
+				default: '',
+				displayOptions: {
+					show: {
+						hasOutputParser: [true],
+					},
+				},
+			},
 		],
 	};
 
@@ -413,10 +441,14 @@ export class ChainLlm implements INodeType {
 			0,
 		)) as BaseLanguageModel;
 
-		const outputParsers = (await this.getInputConnectionData(
-			NodeConnectionType.AiOutputParser,
-			0,
-		)) as BaseOutputParser[];
+		let outputParsers: BaseOutputParser[] = [];
+
+		if (this.getNodeParameter('hasOutputParser', 0, true) === true) {
+			outputParsers = (await this.getInputConnectionData(
+				NodeConnectionType.AiOutputParser,
+				0,
+			)) as BaseOutputParser[];
+		}
 
 		for (let itemIndex = 0; itemIndex < items.length; itemIndex++) {
 			const prompt = this.getNodeParameter('prompt', itemIndex) as string;