fix: scope wallet reservations to draft invoices

[my-dev-jour]

Summary

• only reserve wallet balance for draft POS invoices, not submitted invoices already reflected in GL
• scope pending wallet reservations to the same company as the wallet lookup
• add regression coverage for both wallet helper implementations

Problem

Wallet balance lookup subtracted pending wallet payments using all POS invoices with docstatus in [0, 1] and without company scoping.

That meant a submitted POS invoice could reduce available wallet balance a second time even though its effect was already in GL, and reservations could leak across companies.

Staging Reproduction

Confirmed on a staging environment (ERPNext 16.11.0 / POSNext 1.15.0):

• mocked wallet GL balance to 300
• a submitted partial POS invoice with outstanding 100 and a wallet-flagged payment reduced available balance from 300 to 200
• this proved submitted invoices were being subtracted again as pending reservations

Fix

• change pending reservation queries to draft invoices only (docstatus = 0)
• pass company through wallet balance and available balance calculations
• keep exclude_invoice behavior intact

Verification

• staging reproduction confirmed the original bug
• local verification was limited to py_compile in this workspace