feat: add piece_count and piece_size fields to BomItem for cut-to-length parts (#12422)

* feat: add piece_count and piece_size fields to BomItem for cut-to-length parts

Manufacturing BOMs frequently require multiple pieces of a specific size
cut from continuous stock (cables, tubing, structural profiles). Currently
the only way to express "10 pieces of 250mm cable" is to enter the total
length (2.5m) as quantity, which loses the piece-count information that
purchasing and production need.

This adds two optional fields to BomItem:
- piece_count: number of discrete pieces required (default: 1)
- piece_size: size/length of each piece (e.g. "250 mm")

When piece_size is specified, the total quantity is auto-calculated as
piece_count × piece_size, maintaining full backward compatibility (existing
items effectively have piece_count=1 and empty piece_size).

Changes:
- Backend: new model fields, migration, updated recalculate_quantity()
  logic, hash_fields for BOM validation
- API: serializer exposes piece_count and piece_size
- Frontend: BOM form includes the new fields, BOM table shows them as
  optional columns

Addresses #10274

* refactor: simplify to single piece_count field per reviewer feedback

Remove the piece_size field entirely. The existing quantity field already
represents the per-piece size/length, so piece_count multiplied by
quantity gives the total material requirement.

Example: quantity=200mm, piece_count=10 → total 2m of wire in 10 pieces.

Changes:
- Remove piece_size model field, serializer field, and frontend column/form
- Update migration to only add piece_count
- Update get_required_quantity() to multiply by piece_count
- Restore original recalculate_quantity() without piece_size logic

* test/docs: add unit tests and documentation for piece_count field

* style: replace ambiguous × with x to fix RUF002 lint error

* Address review feedback: api_version bump, changelog, style fix

- Bump INVENTREE_API_VERSION to 531 with entry for piece_count field
- Add CHANGELOG.md entry under Unreleased > Added
- Fix RUF001: replace ambiguous × with x in serializers.py help_text

* fix: align piece_count migration help_text with model (RUF001)

The 0153 AddField recorded help_text with a Unicode multiplication sign
(×), while the model field uses plain 'x' after the RUF001 fix. This
mismatch made makemigrations --check flag an unstaged
0154_alter_bomitem_piece_count migration, failing the DB test CI jobs.

Update the original migration's help_text (and docstring) to plain 'x'
so the field definition matches the model, keeping a single clean
migration instead of add-then-alter.

* fix: use set_quantity() in piece_count tests

BomItem.quantity is a derived field, recalculated from raw_amount on
every save() via recalculate_quantity(). Setting item.quantity directly
was overwritten back to the fixture value on save, so the tests computed
against quantity=3 and failed. Use set_quantity() (which sets raw_amount)
to match how quantity is meant to be updated.

* ci: re-trigger CI to confirm Firefox E2E failures are transient

---------

Co-authored-by: Aman Jain <jainamn@amazon.com>

CHANGELOG.md

@@ -28,6 +28,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- [#12422](https://github.com/inventree/InvenTree/pull/12422) adds a "piece_count" field to the BomItem model, for representing cut-to-length parts. The total material required is calculated as `quantity x piece_count`.
 - Adds configurable default for merging purchase order line items via the `PURCHASEORDER_MERGE_LINE_ITEMS` global setting
 - [#12393](https://github.com/inventree/InvenTree/pull/12393) adds "discount" attribute to order line items, allowing users to specify a discount for each line item on an order. The discount can be specified as either a percentage or a fixed amount, and is applied to the line item total when calculating the order total.
 - [#12391](https://github.com/inventree/InvenTree/pull/12391) adds facility for bulk deleting line items against orders