refactor(core): add text stream parser (#12459)

Support [AI-82](https://linear.app/affine-design/issue/AI-82).

Added a `TextStreamParser` class to standardize formatting of different types of AI stream chunks across providers.

### What changed?

- Created a new `TextStreamParser` class in `utils.ts` that handles formatting of various chunk types (text-delta, reasoning, tool-call, tool-result, error)
- Refactored the Anthropic, Gemini, and OpenAI providers to use this shared parser instead of duplicating formatting logic
- Added comprehensive tests for the new `TextStreamParser` class, including tests for individual chunk types and sequences of chunks
- Defined a common `AITools` type to standardize tool interfaces across providers

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit

- **New Features**
	- Enhanced formatting and structure for streamed AI responses, including improved handling of callouts, web search, and web crawl results.
- **Refactor**
	- Streamlined and unified the processing of streamed AI response chunks across providers for more consistent output.
- **Bug Fixes**
	- Improved error handling and display for streamed responses.
- **Tests**
	- Added comprehensive tests to ensure correct formatting and handling of various streamed message types.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/plugins/copilot/providers/gemini/gemini.ts

@@ -25,7 +25,7 @@ import type {
   PromptMessage,
 } from '../types';
 import { ModelOutputType } from '../types';
-import { chatToGPTMessage } from '../utils';
+import { chatToGPTMessage, TextStreamParser } from '../utils';
 
 export const DEFAULT_DIMENSIONS = 256;
 
@@ -37,8 +37,6 @@ export type GeminiConfig = {
 export abstract class GeminiProvider<T> extends CopilotProvider<T> {
   private readonly MAX_STEPS = 20;
 
-  private readonly CALLOUT_PREFIX = '\n> [!]\n> ';
-
   protected abstract instance:
     | GoogleGenerativeAIProvider
     | GoogleVertexProvider;
@@ -167,42 +165,13 @@ export abstract class GeminiProvider<T> extends CopilotProvider<T> {
         },
       });
 
-      let lastType;
-      // reasoning, tool-call, tool-result need to mark as callout
-      let prefix: string | null = this.CALLOUT_PREFIX;
+      const parser = new TextStreamParser();
       for await (const chunk of fullStream) {
-        if (chunk) {
-          switch (chunk.type) {
-            case 'text-delta': {
-              let result = chunk.textDelta;
-              if (lastType !== chunk.type) {
-                result = '\n\n' + result;
-              }
-              yield result;
-              break;
-            }
-            case 'reasoning': {
-              if (prefix) {
-                yield prefix;
-                prefix = null;
-              }
-              let result = chunk.textDelta;
-              if (lastType !== chunk.type) {
-                result = '\n\n' + result;
-              }
-              yield this.markAsCallout(result);
-              break;
-            }
-            case 'error': {
-              const error = chunk.error as { type: string; message: string };
-              throw new Error(error.message);
-            }
-          }
-          if (options.signal?.aborted) {
-            await fullStream.cancel();
-            break;
-          }
-          lastType = chunk.type;
+        const result = parser.parse(chunk);
+        yield result;
+        if (options.signal?.aborted) {
+          await fullStream.cancel();
+          break;
         }
       }
     } catch (e: any) {
@@ -222,10 +191,6 @@ export abstract class GeminiProvider<T> extends CopilotProvider<T> {
     return result;
   }
 
-  private markAsCallout(text: string) {
-    return text.replaceAll('\n', '\n> ');
-  }
-
   private isReasoningModel(model: string) {
     return model.startsWith('gemini-2.5');
   }