refactor: extract shared FmcApiError base class from MarginApiError/PositionsError (#366)

Both MarginApiError (lib/margin-api.js) and PositionsError (lib/positions.js)
shared an identical constructor pattern: extend Error, set this.type, set this.name.

Extract the shared pattern into FmcApiError (lib/api-error.js):
- this.type is set once in FmcApiError constructor
- this.name is set via this.constructor.name, picking up 'MarginApiError' or
  'PositionsError' automatically in each subclass
- Error.cause chaining is inherited unchanged

The two subclasses become empty one-liners, removing 14 lines of identical code.
Any future addition to the shared constructor (e.g. a new field, a new guard)
now only requires one edit rather than two identical changes in separate files.

Add lib/api-error.js before lib/positions.js and lib/margin-api.js in manifest.json
so FmcApiError is defined before both modules load.

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

lib/api-error.js

@@ -0,0 +1,35 @@
+// Shared error base class for FMC API client modules.
+// Both MarginApiError (lib/margin-api.js) and PositionsError (lib/positions.js)
+// follow the same pattern: extend Error, set this.type (one of FMC_CONSTANTS.ERROR_TYPES),
+// and support Error.cause chaining for stack-trace preservation.
+// Centralised here so adding new properties or changing the constructor signature only
+// requires one edit rather than two identical changes across both API modules.
+'use strict';
+
+/**
+ * Base class for typed API errors thrown by FMC's API client modules.
+ * Subclasses (`MarginApiError`, `PositionsError`) inherit the `type` field and
+ * Error.cause support; `this.name` is set automatically from the subclass constructor name.
+ * Subclasses need no constructor of their own — they inherit this one, which sets
+ * `this.name` to the subclass name via `this.constructor.name`.
+ *
+ * @extends Error
+ */
+class FmcApiError extends Error {
+  /**
+   * @param {string} message - Human-readable error description.
+   * @param {string} type - One of `FMC_CONSTANTS.ERROR_TYPES`.
+   * @param {ErrorOptions} [options] - Standard Error options; pass `{ cause: originalError }`
+   *   to chain the original fetch/parse exception so DevTools shows the full error stack.
+   *   Supported in Chrome 93+ (well within the manifest's minimum_chrome_version: 111).
+   */
+  constructor(message, type, options) {
+    super(message, options);
+    this.type = type;
+    // Set this.name to the concrete subclass name so stack traces read
+    // 'MarginApiError' or 'PositionsError' rather than the generic 'Error'.
+    // this.constructor.name resolves correctly for named class expressions.
+    // No minification is applied (extension loaded unpacked), so names are stable.
+    this.name = this.constructor.name;
+  }
+}

lib/margin-api.js

@@ -220,20 +220,9 @@ fragment ImageDetails on Image {
     return ErrorType.CLIENT_ERROR;
   }
 
-  class MarginApiError extends Error {
-    /**
-     * @param {string} message - Human-readable error description.
-     * @param {string} type - One of `FMC_CONSTANTS.ERROR_TYPES`.
-     * @param {ErrorOptions} [options] - Standard Error options; pass `{ cause: originalError }`
-     *   to chain the original fetch/parse exception so DevTools shows the full error stack.
-     *   Supported in Chrome 93+ (well within the manifest's minimum_chrome_version: 111).
-     */
-    constructor(message, type, options) {
-      super(message, options);
-      this.type = type;
-      this.name = 'MarginApiError';
-    }
-  }
+  // Inherits constructor, this.type assignment, and this.name = this.constructor.name
+  // from FmcApiError (lib/api-error.js). No constructor needed here.
+  class MarginApiError extends FmcApiError {}
 
   const debugLog = makeDebugLog('[FMC-API]');
 

lib/positions.js

@@ -87,20 +87,9 @@ const PositionsAPI = (() => {
   // RETRYABLE_TYPES) rather than being accessed via FMC_CONSTANTS inside doFetchPriceList.
   const MAX_RETRY_AFTER_MS = FMC_CONSTANTS.MAX_RETRY_AFTER_MS;
 
-  class PositionsError extends Error {
-    /**
-     * @param {string} message - Human-readable error description.
-     * @param {string} type - One of `FMC_CONSTANTS.ERROR_TYPES`.
-     * @param {ErrorOptions} [options] - Standard Error options; pass `{ cause: originalError }`
-     *   to chain the original fetch/parse exception so DevTools shows the full error stack.
-     *   Supported in Chrome 93+ (well within the manifest's minimum_chrome_version: 111).
-     */
-    constructor(message, type, options) {
-      super(message, options);
-      this.type = type;
-      this.name = 'PositionsError';
-    }
-  }
+  // Inherits constructor, this.type assignment, and this.name = this.constructor.name
+  // from FmcApiError (lib/api-error.js). No constructor needed here.
+  class PositionsError extends FmcApiError {}
 
   /**
    * Converts a raw portfolio position object from the portfolio GraphQL API into

manifest.json

@@ -29,6 +29,7 @@
         "lib/debug.js",
         "lib/retry.js",
         "lib/fetch-utils.js",
+        "lib/api-error.js",
         "lib/positions.js",
         "lib/margin-api.js",
         "lib/margin-calc.js",