Charge legacy us_bank_account instruments through PaymentIntents

[asterdrak]

Related work:
https://github.com/maxio-com/chargify/pull/30281

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Reject a mismatched instrument before singleton fallback

When unstore receives an explicit compound identifier such as cus_X|pm_old, but Stripe now lists one different instrument—for example, because pm_old was already detached or replaced—this fallback silently selects and detaches that remaining instrument. This can remove another payment profile's live bank account even though the caller named a different one; the singleton fallback should apply only when instrument_id is absent, while a supplied but unlisted ID should produce a no-op or error.

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[asterdrak]

Hardened in 8cbff29: a named instrument that is no longer attached now returns a success no-op ("Payment method already detached") instead of falling through to the singleton/default guesses, with a test. Note the scenario is unreachable through the only production caller today — conduit passes the bare cus_ token, never a compound id — so this closes the API surface, not a live path.

[asterdrak]

Test evidence, since this repo has no CI:

• Gateway under change: test/unit/gateways/stripe_ach_setup_intents_test.rb — 22 tests, 71 assertions, 0 failures at head 8cbff291.
• Full unit suite, head vs merge-base (a466b20e): 3,616 tests both; identical failure sets (62) — the diff between the two runs is a single object memory address. Every failure, including all 12 in StripeTest, pre-exists on master (Ruby 3.4 bit-rot: Psych 4 strictness, extract_options!, test-unit skip) and is untouched by this PR.
• Integration: maxio-com/chargify#30281 CI runs the full app suite with Gemfile pinned to this branch's head, which exercises this gateway through conduit.
• Real-API coverage: the staging end-to-end on PAY-3931 (store → mandate → PaymentIntent charge on a live legacy ba_); the one deliberately unexecuted call is PaymentMethod.detach against a legacy ba_ on a real account, which is why profile destroy stays on the legacy vault for the migrated book.

[dbackowski]

Small refactoring here: #258

[dbackowski]

🚀