chore: devoid pr of indirections

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Context

• pr.flags.mute_email for ad-hoc silencing (no current use case)
• Use already existing, but forgotten pr.mute_email for tx-wise silencing (shopping cart flow)
• Use already existing, but forgotten pr.make_sales_invoice for the control flow to create an invoice upon payment (shopping cart flow)

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Detected while working on frappe/payments#53

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@ruthra-kumar @s-aga-r Since I'm sprinting on frappe/payments#53 to make payments more reliable in ERPNext (the current situation in production is brittle), this PR came up among the following more:

• fix: tests with payment gateway #40774
• fix(pr): set advace Payment status based on Payment Request status #40643
• and crucially: feat(payment): add more payment channels than just email and phone (think: chat) #40424

It would help me a lot if you can afford in some form or shape to "mirror" this sprint in the sense to review / merge acceptable increments somewhat swiftly.

My end goal remains to robustly implement PayZen atop frappe/payments#53 and face-lift the payment infrastructure so that implementing new payment gateways becomes more reliable and (hopefully also: easier) for the greater community.

The current situation is not really tenable and source of a major maintenance headache in our production deployment. We're iterating on this for the 3rd time, now, and the legacy spaghetti (I hope this is an undisputed claim 😄) really didn't help me nor the business bottom line. 😢

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Now part of #40424 with better test coverage