customHeaders are ignored by the primary request path used by table/record methods

[nouraellm]

Description

customHeaders passed to the Airtable constructor (or via options) are not applied to the vast majority of requests made by the library.

Root cause

• Base.makeRequest() correctly merges this._airtable._customHeaders via _getRequestHeaders().
• Almost all high-level API methods (table.select(), .create(), .update(), .replace(), .destroy(), .find(), and the corresponding methods on Record) go through the older runAction path (src/run_action.ts).
• run_action.ts hardcodes the headers and never reads _customHeaders:

const headers = {
    authorization: `Bearer ${base._airtable._apiKey}`,
    'x-api-version': base._airtable._apiVersion,
    'x-airtable-application-id': base.getId(),
    'content-type': 'application/json',
};