Legacy capture webhook refactoring

[garmashdev]

Description

Refactoring the invoice webhook for the legacy flow to align it with the API flow:
https://github.com/BoltApp/source/blob/5fd05c30b2518b8d813be44b4815d8c0a2807f7f/hail/services/merchantproxy/cart/magento2/webhook.go#L620

Changes introduced in this PR:

1. Handling fully covered offline invoices:

• Currently, if an offline invoice fully covering the order is already created in Magento (offline mode or via an ERP system), we throw the error: "The same capture amount was already invoiced."
• In this PR, we simply skip creating a new invoice and avoid throwing an error.

2. Handling partially covered offline invoices:

• Currently, if an offline invoice partially covering the order is already created in Magento (offline mode or via an ERP system) and a request is made for a full invoice, we throw the error:
  "Capture amount is invalid: capture amount [%1], previously captured [%2], grand total [%3]."
• In this PR, we create an invoice for the remaining amount needed to cover the order instead of throwing an error.

Fixes:
https://app.devrev.ai/bolt-inc/works/ISS-200
https://app.asana.com/0/1200879031426307/1207325055456921/f

#changelog Legacy capture webhook refactoring

Type of change

• Bug fix (change which fixes an issue)
• New feature (change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please validate that you have tested your change in at least one of the following areas:

• Successfully tested locally (or docker image)
• Successfully tested on a staging or sandbox server
• Successfully tested on a merchant's staging server

For PR Reviewer

• Reviewed unit tests to make sure we are using real components rather than mocks as much as possible?
• For any major change (observer, new Bolt feature, core Magento interaction) we must add a feature switch, did you verify this?

Checklist:

• My code follows the style guidelines of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• New and existing unit tests pass locally with my changes.
• I have created or modified unit tests to sufficiently cover my changes.
• I have added my ticket link and provided a changelog message.

[garmashdev]

• Legacy capture webhook refactoring #1908 👈 (View in Graphite)
• master

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