fix(netsuite): skip sync of line-less invoices

[lago-claude-ai-agent]

Finalized invoices whose fees are all zero-amount were still posted to NetSuite, which drops zero-amount lines, ends up with no line item, and rejects the transaction with a 424. That status counts as retryable, so the job retried three times and sent a sync_error webhook on each attempt for an invoice with nothing billable to sync.

Invoice payloads now expose a sync_allowed? predicate defaulting to true, and CreateService returns early (as a success, not an error) when it is false. NetSuite overrides it to mirror the lines that actually survive on its side — positive fees or discounts. Xero, Anrok and HubSpot keep the default, so Xero's deliberate zero-amount line syncing is unchanged. This also makes a manual invoice sync a clean no-op for these invoices.

Deliberately out of scope: the 424 retryable classification stays as is, since the request is no longer sent at all.

[lago-claude-ai-agent]

PASS — the guard matches the lines that actually survive on NetSuite's side, and it is scoped to NetSuite only, so no other provider changes behavior.

Verified:

• Netsuite#sync_allowed? mirrors body's fee_items + discounts: when no positive fee exists, fees falls back to all-zero lines that NetSuite drops, leaving only discounts — so positive_fees.exists? || discounts.any? is exactly "at least one line survives".
• Xero, Anrok and HubSpot inherit the true default and none override sync_allowed?, so Xero's deliberate zero-amount line syncing (fix(intergrations): Fix fees in invoices payload #2656) is untouched.
• No sibling write path was missed: invoice sync to accounting providers only goes through Invoices::CreateService (there is no invoice UpdateService; ReconcileService is a by-tranid GET). The SyncIntegrationInvoice mutation calls the same service, so a manual sync is a no-op too.
• The guard sits after the integration_invoice check and before throttle!, and returns a plain success like the sync_invoices guard above it — no error webhook, no throttle slot burned.
• Memoizing payload is safe: fee_items rebuilds its hashes per call, so nothing carries over between sync_allowed? and body.
• The CreateService spec is a genuine regression test — the post_with_response mock is set up to succeed, so without the guard the request would be sent and the assertion would fail.

Nits (non-blocking):

• positive_fees is a fresh relation each call, so sync_allowed? adds one EXISTS query before the sync; discounts is likewise recomputed once more (cheap — the collection mappings come from an already-loaded association).
• The default-true test instantiates BasePayload directly; asserting it on Xero would tie the guarantee to the class that actually runs in production.