fix: correct defects found while testing on version-16

[av-dev2]

Defects found while building the version-16 test suite. These are logic errors rather than framework drift, so they are separated from the compatibility work. Each commit covers one file.

Fixed

• Batch splitting and item info silently found nothing. custom_api read Stock Ledger Entry.batch_no, which v16 no longer writes now that batches go through bundles. It now reads batch quantities through the stock helpers.
• Six whitelisted endpoints returned data with no permission check (get_item_prices, get_item_prices_custom, get_item_prices_po, get_item_prices_custom_po, get_pending_sales_invoice, get_item_info). They now call frappe.has_permission(..., throw=True). The employee contact QR endpoint had the same gap.
• create_delivery_note_for_all_pending_sales_invoice raised on every daily run — a fiters= typo in the scheduled job.
• Purchase Order budget validation called the Purchase Invoice handler, which reads doc.posting_date; a Purchase Order has no such field, so enabling the check raised AttributeError.
• Import tracker never completed. A fully paid tracker stayed "Active" because totals and status were not recomputed after submit, and computed child fields were not writable after submit.
• Vehicle sync tasks were never claimed — queue.claim_batch referenced an undefined WORKER_ID, so every claim failed and was swallowed by the surrounding except.
• KCB payment files were attached twice on every save, because the File records carried no attached_to_field.
• Stanbic Payments Initiation could not be inserted: two child fields fetched employee.bank_country_code and employee_country_code, columns that do not exist.
• create_the_stock_entry_type raised DuplicateEntryError when re-run, and the Selcom client referenced an unbound response on failure.

Verification

Each fix has a test in the companion tests branch; the full suite is 631 passing test cases on Frappe 16 / ERPNext 16 / HRMS 16.