[PM-37599] refactor: Remove PM37597 always-enable-Stripe-automatic-tax feature flag

[amorask-bitwarden]

🎟️ Tracking

Jira: PM-37599 — final story of epic PM-36315 "Simplify tax logic". Predecessors shipped in 2026.6.0: PM-37597 (#7662), PM-37598 (#7677), PM-37820 defect (#7719).

📔 Objective

Removes the PM37597_AlwaysEnableStripeAutomaticTax feature flag (LaunchDarkly key pm-37597-always-enable-stripe-automatic-tax) and deletes the now-dead flag-OFF tax logic, making the flag-ON behavior unconditional. The flag has been at 100% in production since 2026.6.0, so this is a pure code-debt removal with no end-user behavior change.

What this locks in

• Stripe is the source of truth for taxability; new customers default to tax_exempt = "none" (taxable) and automatic_tax stays enabled.
• Engineering no longer programmatically writes tax_exempt, so a CS-set exemption in Stripe is never overwritten.
• The tax-ID banner is gated on Stripe's view of taxability; US customers never see it (preserves PM-37820).

Changes

• Delete the 11 flag-OFF tax_exempt write branches across the org/provider/premium/payment commands, UpcomingInvoiceHandler, and ProviderBillingService (keeping the already-unconditional automatic_tax writes).
• Collapse the tax-ID banner gating in GetOrganizationWarningsQuery and GetProviderWarningsQuery to the flag-ON conditions.
• Delete TaxHelpers; relocate the {US, CH} postal-code check onto OrganizationCreateRequestModel as RequiresBillingPostalCode (runtime behavior unchanged).
• Inline and remove GetCustomerWhileEnsuringCorrectTaxExemptionAsync (its reconciliation is dead once the flag is always-true).
• Remove the now-unused IFeatureService dependency from 7 classes and delete the flag constant.

No contract or data-model change — the warnings response shape and banner conditions are exactly what production already serves. Server-only, V±2 safe.

Post-merge ops (outside this repo): archive the LaunchDarkly flag pm-37597-always-enable-stripe-automatic-tax.

[github-actions]

🤖 Bitwarden Claude Code Review

Overall Assessment: APPROVE

This is a pure code-debt removal of the PM37597_AlwaysEnableStripeAutomaticTax feature flag, which has been at 100% in production since 2026.6.0. The change deletes the dead flag-OFF tax_exempt write branches across the org/provider/premium/payment commands, UpcomingInvoiceHandler, and ProviderBillingService, collapses the tax-ID banner gating in both warnings queries to the flag-ON conditions, and relocates the {US, CH} postal-code check from the deleted TaxHelpers onto OrganizationCreateRequestModel.RequiresBillingPostalCode. Tests were updated in lockstep with the removed branches, and no runtime behavior changes for end users.

Code Review Details

No blocking findings. Verification performed:

• Both GetOrganizationWarningsQuery and GetProviderWarningsQuery preserve the exact flag-ON conditions (TaxExempt != None → null, US country → null).
• The GetCustomerWhileEnsuringCorrectTaxExemptionAsync inlining matches the prior flag-ON path (plain GetCustomerOrThrow with the same ["tax", "tax_ids"] expansions).
• RequiresBillingPostalCode is a faithful relocation of TaxHelpers.IsDirectTaxCountry — same {US, CH} set and same null/empty guard, so runtime behavior is unchanged.
• No dangling references remain to TaxHelpers, IsDirectTaxCountry, DetermineTaxExemptStatus, the removed flag key, or any inlined/removed helper method (verified repo-wide).
• Removed IFeatureService dependencies and the flag constant are consistently dropped across the 7 affected classes and their tests.

[r-tome]

postalCodeRequired is the expected outcome so I'd prefer if this was split into two tests, one expecting true other for false.