refactor(javascript): MessageProcessor abstraction added and implemented in catcher event processing pipeline

- MessageProcessor interface and MessageHint type
- BrowserMessageProcessor, BreadcrumbsMessageProcessor, ConsoleCatcherMessageProcessor, DebugMessageProcessor
- replaced inline addon logic with sequential MessageProcessor pipeline

Author: Reversean

packages/core/src/index.ts

@@ -13,3 +13,4 @@ export { buildElementSelector } from './utils/selector';
 export { EventRejectedError } from './errors';
 export { isErrorProcessed, markErrorAsProcessed } from './utils/event';
 export type { BreadcrumbStore, BreadcrumbsAPI, BreadcrumbHint, BreadcrumbInput } from './breadcrumbs/breadcrumb-store';
+export type { MessageHint, MessageProcessor } from './messages/message-processor'

packages/core/src/messages/message-processor.ts

@@ -0,0 +1,36 @@
+import { Breadcrumb, CatcherMessagePayload, CatcherMessageType } from '@hawk.so/types';
+
+/**
+ * Snapshot of event context captured synchronously at error time,
+ * before any processing.
+ */
+export interface MessageHint {
+  /**
+   * Original caught error.
+   */
+  error?: Error | string;
+
+  /**
+   * Breadcrumbs captured at error time.
+   */
+  breadcrumbs?: Breadcrumb[];
+}
+
+/**
+ * Single step in message processing pipeline before message is sent.
+ *
+ * @typeParam T - catcher message type this processor handles
+ */
+export interface MessageProcessor<T extends CatcherMessageType = CatcherMessageType> {
+  /**
+   * Handles input message. May mutate or replace it.
+   *
+   * @param payload - processed event message payload
+   * @param hint - additional context about original error
+   * @returns modified payload, or `null` to drop event
+   */
+  apply(
+    payload: CatcherMessagePayload<T>,
+    hint?: MessageHint,
+  ): CatcherMessagePayload<T> | null
+}