[Backport version-15] feat: book port expenses to a work in progress account

[github-actions]

Backport of #484 to version-15.

---

Why

Port expenses are bought against a container and carried on the ICD Container, ICD Master BL and Manifest dimensions, but they post straight to an expense account, so the cost hits the profit and loss before the container has been invoiced. The ICD wants them held in work in progress and released to cost of goods sold when the container is billed.

This is the first half — holding them on WIP. The release to COGS is a separate change.

Settings

A new Expense Accounting section at the top of the Expenses tab in ICD TZ Settings:

field	behaviour
enable_wip_for_expenses — Enable WIP for Port Expenses	Check, off by default
wip_account	Link to Account, shown and mandatory only while the check is on
cogs_account	Link to Account, shown and mandatory only while the check is on

The form offers only postable accounts of the user's company. The server refuses a save where either account is missing, is a group account, where the two are the same (releasing an expense would post it to itself), or where they belong to different companies — because the form filter does not reach an import, a patch or the API, and a group account otherwise fails far later at posting time.

Purchase Order

build_purchase_order resolves the account once and every line carries it.

setting	expense account on the saved line
off	Cost of Goods Sold - _TC7 — whatever ERPNext chooses
on	Expenses Included In Asset Valuation - _TC7 — the WIP account

With the setting off the field is left unset, which is indistinguishable from how it behaved before: ERPNext fills it from the item and company defaults.

Purchase Invoice

expense_account already copies from order to invoice — frappe's mapper copies same-named fields — but an invoice raised on its own has none, and any invoice can be edited afterwards. A line that slipped onto another account would never be released.

A before_validate hook holds the line on WIP. Verified: an invoice raised on its own, with its line starting on another account, was moved onto WIP.

Only a configured port expense item on a container line is touched. A container can be tagged on any purchase, and the rest of that invoice is ERPNext's to account for — a stock item under perpetual inventory above all, whose Stock Received But Not Billed account must not be overwritten. Running before validate also leaves ERPNext's own assignments intact and lets against_expense_account name what the invoice actually posts to.

Multi-company

An account belongs to one company and the settings hold one account. Another company's documents are therefore left alone rather than made unsavable — they keep ERPNext's default. The exception is where the ICD is deliberately creating a port expense order: there a configured account that cannot serve the company is reported, because silence would send the expense to the default and it would never be released.

If expenses are genuinely booked in more than one company, the accounts need to be held per company. Worth settling alongside the COGS release.

Worth knowing before this is enabled

cogs_account is captured but nothing reads it yet. Until the release is built, ticking this accumulates a WIP balance that nothing clears.

Testing

icd_tz/tests/test_expense_wip_account.py — the account chosen and left alone by the setting; an order refused when the configured account cannot serve its company while an incidental document is left alone; the settings refusing a missing, group, repeated or cross-company account; an order line carrying the account and carrying none when off; and an invoice holding only a configured expense line on a container, leaving a non-expense item, a line with no container, and everything else alone.

Also checked end to end on a saved Purchase Order and an invoice, not only in memory.

Full suite: 310 passing. pre-commit run --all-files and ruff check green.