fix: Add non-existent Item check and cleanup in validate_for_items

[marination]

Issue:

Traceback (most recent call last):
  File "apps/frappe/frappe/app.py", line 68, in application
    response = frappe.api.handle()
  File "apps/frappe/frappe/api.py", line 55, in handle
    return frappe.handler.handle()
  File "apps/frappe/frappe/handler.py", line 31, in handle
    data = execute_cmd(cmd)
  File "apps/frappe/frappe/handler.py", line 68, in execute_cmd
    return frappe.call(method, **frappe.form_dict)
  File "apps/frappe/frappe/__init__.py", line 1213, in call
    return fn(*args, **newargs)
  File "apps/frappe/frappe/desk/form/save.py", line 21, in savedocs
    doc.save()
  File "apps/frappe/frappe/model/document.py", line 287, in save
    return self._save(*args, **kwargs)
  File "apps/frappe/frappe/model/document.py", line 309, in _save
    return self.insert()
  File "apps/frappe/frappe/model/document.py", line 240, in insert
    self.run_before_save_methods()
  File "apps/frappe/frappe/model/document.py", line 971, in run_before_save_methods
    self.run_method("validate")
  File "apps/frappe/frappe/model/document.py", line 869, in run_method
    out = Document.hook(fn)(self, *args, **kwargs)
  File "apps/frappe/frappe/model/document.py", line 1161, in composer
    return composed(self, method, *args, **kwargs)
  File "apps/frappe/frappe/model/document.py", line 1144, in runner
    add_to_return_value(self, fn(self, *args, **kwargs))
  File "apps/frappe/frappe/model/document.py", line 866, in fn
    return method_object(*args, **kwargs)
  File "apps/erpnext/erpnext/buying/doctype/purchase_order/purchase_order.py", line 62, in validate
    validate_for_items(self)
  File "apps/erpnext/erpnext/buying/utils.py", line 65, in validate_for_items
    d.item_code, as_dict=1)[0]
IndexError: list index out of range

Fix:

• Added a validation if invalid item code is passed via data import/API, etc.
• Used DB APIs instead of raw sql
• Separated checks into separate functions
• Added return types to functions
• Better variable naming and removed redundant utils import in function

[marination]

skipping tests because this is a defensive fix. Could not replicate this

[codecov]

Codecov Report

Merging #30509 (93f6346) into develop (b981fae) will increase coverage by 8.57%.
The diff coverage is 92.00%.

@@             Coverage Diff             @@
##           develop   #30509      +/-   ##
===========================================
+ Coverage    52.28%   60.86%   +8.57%     
===========================================
  Files         1082     1082              
  Lines        69067    69076       +9     
===========================================
+ Hits         36114    42043    +5929     
+ Misses       32953    27033    -5920     

Impacted Files	Coverage Δ
erpnext/buying/utils.py	75.80% <92.00%> (+4.10%)	⬆️
...rial_no_valuation/incorrect_serial_no_valuation.py	85.96% <0.00%> (-10.53%)	⬇️
...rpnext/accounts/doctype/shareholder/shareholder.py	80.00% <0.00%> (-10.00%)	⬇️
...urity_shortfall/process_loan_security_shortfall.py	93.75% <0.00%> (-6.25%)	⬇️
...eorder_level/itemwise_recommended_reorder_level.py	88.88% <0.00%> (-3.71%)	⬇️
erpnext/hr/doctype/job_applicant/job_applicant.py	63.93% <0.00%> (-3.28%)	⬇️
...e_sales_analytics/supplier_wise_sales_analytics.py	86.88% <0.00%> (-3.28%)	⬇️
erpnext/stock/doctype/warehouse/warehouse.py	82.78% <0.00%> (-1.64%)	⬇️
.../report/stock_projected_qty/stock_projected_qty.py	86.84% <0.00%> (-1.32%)	⬇️
.../support/report/issue_analytics/issue_analytics.py	88.98% <0.00%> (-0.85%)	⬇️
... and 177 more