fix(core): allow Authorize without payment_method on the 3DS return leg

[shuklatushar226]

Problem

PaymentService::authorize required payment_method unconditionally:

proto_payload.payment_method.clone().ok_or(MissingRequiredField { field_name: "payment_method" })?

Connectors that finalise 3-D Secure through the Authorize flow — Ilixium, NMI, Airwallex,
Nexixpay and others that key off request.redirect_response.is_some() — are re-invoked after
the ACS posts back to TermUrl. At that point the caller no longer holds the raw instrument, so
Hyperswitch sends payment_method_data: None and omits the proto field (it is optional in both
the proto and Hyperswitch's CompleteAuthorizeData).

UCS therefore rejected the completion call with InvalidArgument: Missing required field: payment_method before any connector code ran, making 3DS completion impossible for every
connector that uses this pattern.

Fix

payment_method is now required only when the call is not a redirect-return leg. The leg is
identified by redirection_response, the same signal the connectors themselves already use.
A plain authorize with no payment_method is still rejected exactly as before.

PaymentsAuthorizeData::payment_method_data is not optional, so the absent instrument is
modelled explicitly as a new PaymentMethodData::NoInstrumentAfterRedirect variant rather than
substituted with a default (e.g. MandatePayment) that a connector could mistake for a payable
instrument.

Adding the variant is compiler-enforced: all 51 existing matches on PaymentMethodData now
handle it through their existing unsupported-payment-method rejection arm, so no connector's
behaviour changes and a connector that later wants to accept this leg has to opt in deliberately.
That accounts for the file count — the connector diffs are one added pattern alternative each.

PaymentsAuthorizeData::foreign_try_from derived payment_method_type from the same field and
had the same unconditional gate; it now returns None for the instrument-less leg only (matched
on NoInstrumentAfterRedirect), and errors as before otherwise.

Verification

Driven end to end against a mock Ilixium server (Hyperswitch → UCS → mock):

• 3DS: POST /payments → requires_customer_action → ACS challenge → complete-authorize →
  mock received POST /platform/ili/direct/threedcomplete (merchant digest verified) → HS
  payment status succeeded. This call was rejected with InvalidArgument before the fix.
• Direct gRPC: PaymentService/Authorize with payment_method omitted and
  redirection_response present now reaches the connector and returns CHARGED.
• No-3DS regression: unchanged, payment succeeded.
• Validation preserved: Authorize with no payment_method and no redirection_response
  still fails with InvalidArgument: Missing required field: payment_method.

cargo check --workspace --all-targets, cargo fmt --all and cargo clippy on the touched
crates are clean.

Context

Found while integrating the Ilixium connector (#2125). Raised separately because it is a
core-flow change that unblocks 3DS completion for every UCS connector using this pattern, not
just Ilixium.

🤖 Generated with Claude Code

[shuklatushar226]

Superseded — folding this into the single Ilixium PR #2125 rather than raising a separate PR. The identical change is now on feat/grace-Illexium as commit 664e8ab, kept as its own commit so it stays reviewable in isolation.