fix(amazonq): handle response errors from lsp (aws#7161)

## Problem
Response errors returned by the LSP are returned to chat after
prepending a custom error message. We can remove this custom error
message and rely on flare for a single implementation of this error
message.


## Solution
- perform a sanity check on the response before casting it to the proper
type.
- log the last result from the language server to help debug. 
- include the requestId clearly in the logs. 

<img width="1061" alt="image"
src="https://github.com/user-attachments/assets/fd71ed26-7266-47ee-9204-c1ec412ca452"
/>



---

- Treat all work as PUBLIC. Private `feature/x` branches will not be
squash-merged at release time.
- Your code changes must meet the guidelines in
[CONTRIBUTING.md](https://github.com/aws/aws-toolkit-vscode/blob/master/CONTRIBUTING.md#guidelines).
- License: I confirm that my contribution is made under the terms of the
Apache 2.0 license.

Author: Hweinstock

packages/amazonq/src/lsp/chat/error.ts

@@ -0,0 +1,23 @@
+/*!
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+import { ChatResult } from '@aws/language-server-runtimes/protocol'
+import { ResponseError } from '@aws/language-server-runtimes/protocol'
+/**
+ * Perform a sanity check that the error we got from the LSP can be safely cast to the expected type.
+ * @param error
+ * @returns
+ */
+export function isValidResponseError(error: unknown): error is ResponseError<ChatResult> & { data: ChatResult } {
+    return (
+        typeof error === 'object' &&
+        error !== null &&
+        'code' in error &&
+        typeof error.code === 'number' &&
+        'message' in error &&
+        typeof error.message === 'string' &&
+        'data' in error &&
+        error.data !== undefined
+    )
+}

packages/amazonq/src/lsp/chat/messages.ts

@@ -61,6 +61,7 @@ import { AuthUtil } from 'aws-core-vscode/codewhisperer'
 import { amazonQDiffScheme, AmazonQPromptSettings, messages, openUrl } from 'aws-core-vscode/shared'
 import { DefaultAmazonQAppInitContext, messageDispatcher, EditorContentController } from 'aws-core-vscode/amazonq'
 import { telemetry, TelemetryBase } from 'aws-core-vscode/telemetry'
+import { isValidResponseError } from './error'
 
 export function registerLanguageServerEventListener(languageClient: LanguageClient, provider: AmazonQChatViewProvider) {
     languageClient.info(
@@ -255,22 +256,16 @@ export function registerMessageListeners(
                         chatDisposable
                     )
                 } catch (e) {
-                    languageClient.info(`Error occurred during chat request: ${e}`)
-                    // Use the last partial result if available, append error message
-                    let body = ''
-                    if (!cancellationToken.token.isCancellationRequested) {
-                        body = lastPartialResult?.body
-                            ? `${lastPartialResult.body}\n\n ❌ Error: Request failed to complete`
-                            : '❌ An error occurred while processing your request'
-                    }
-
-                    const errorResult: ChatResult = {
-                        ...lastPartialResult,
-                        body,
+                    const errorMsg = `Error occurred during chat request: ${e}`
+                    languageClient.info(errorMsg)
+                    languageClient.info(
+                        `Last result from langauge server: ${JSON.stringify(lastPartialResult, undefined, 2)}`
+                    )
+                    if (!isValidResponseError(e)) {
+                        throw e
                     }
-
                     await handleCompleteResult<ChatResult>(
-                        errorResult,
+                        e.data,
                         encryptionKey,
                         provider,
                         chatParams.tabId,

packages/amazonq/test/unit/amazonq/lsp/chat/error.test.ts

@@ -0,0 +1,15 @@
+/*!
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { isValidResponseError } from '../../../../../src/lsp/chat/error'
+import { ResponseError } from '@aws/language-server-runtimes/protocol'
+import * as assert from 'assert'
+
+describe('isValidResponseError', async function () {
+    it('requires the data field', function () {
+        assert.ok(isValidResponseError(new ResponseError(0, 'this one has data', {})))
+        assert.ok(!isValidResponseError(new ResponseError(0, 'this one does not have data')))
+    })
+})