fix: restore weighbridge_ticket item link fields dropped by #136

[av-dev2]

Summary

• PR fix: sync authotp and weighbridge custom fields via json fixtures #136 replaced av_tools.weigh_bridge.custom_fields.setup_custom_fields with a JSON fixture but was built against a stale local copy of that file, so it only carried over the Vehicle.default_tare_weight field and silently dropped the weighbridge_ticket Link field on six Item child doctypes (Sales Invoice Item, Delivery Note Item, Sales Order Item, Purchase Order Item, Purchase Invoice Item, Purchase Receipt Item).
• That field is live functionality — weigh_bridge/api.py writes row.weighbridge_ticket / child.weighbridge_ticket when building invoices/orders from a weighbridge ticket — so any new install or resync would no longer create it.
• This was also the root cause of both failed backports of fix: sync authotp and weighbridge custom fields via json fixtures #136: git cherry-pick hit a modify/delete conflict on weigh_bridge/custom_fields.py because version-15/version-16-hotfix carry different content for that file than what fix: sync authotp and weighbridge custom fields via json fixtures #136's deletion commit expected.
• Restores the six fields into weigh_bridge_custom_fields.json, alongside the Vehicle field, so the existing generic create_custom_fields.execute loader recreates them.

Test plan

• Ran av_tools.utils.create_custom_fields.execute against an existing site (amex-local); confirmed all six weighbridge_ticket Item fields plus Vehicle.default_tare_weight are created/synced correctly and tagged with the Av Tools module.
• pre-commit run --all-files passes.

[github-actions]

Backport failed for version-15, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin version-15
git worktree add -d .worktree/backport-137-to-version-15 origin/version-15
cd .worktree/backport-137-to-version-15
git switch --create backport-137-to-version-15
git cherry-pick -x cb3192d4bb1f5aee54db2655e17a21500af08cc6

[github-actions]

Backport failed for version-16-hotfix, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin version-16-hotfix
git worktree add -d .worktree/backport-137-to-version-16-hotfix origin/version-16-hotfix
cd .worktree/backport-137-to-version-16-hotfix
git switch --create backport-137-to-version-16-hotfix
git cherry-pick -x cb3192d4bb1f5aee54db2655e17a21500af08cc6