refactor(proto)!: use Connector enum for Apple Pay session token connector field

[Utkal059]

Description

ApplepayClientAuthenticationResponse.connector was string, even though every caller
fills it in with a connector name that already exists as a variant of the Connector enum
in the same file. This makes it Connector.

Currency, CountryAlpha2 and SdkNextAction are already carried as proto enums on this
message, so the string was the odd one out.

Changes:

• payment.proto — string connector = 3; becomes Connector connector = 3; on
  ApplepayClientAuthenticationResponse.
• domain_types/connector_types.rs — the matching domain field becomes ConnectorEnum.
• braintree / trustpay transformers — the two connectors that build an Apple Pay session
  token now pass ConnectorEnum::Braintree / ConnectorEnum::Trustpay instead of
  BRAINTREE_CONNECTOR_NAME.to_string() / "trustpay".to_string(). Same value at runtime;
  BRAINTREE_CONNECTOR_NAME stays, since the Google Pay and PayPal responses on the same
  message still use it.
• domain_types/types.rs — a new ForeignTryFrom<ConnectorEnum> for
  grpc_api_types::payments::Connector, used by both grpc conversion sites. It resolves
  through the SCREAMING_SNAKE_CASE name exactly like the existing CountryAlpha2
  conversion, and returns UnexpectedResponseError rather than falling back to
  CONNECTOR_UNSPECIFIED, so a missing mapping surfaces as an error instead of an
  unusable session token.

I scoped this to the Apple Pay message to keep it symmetric with the hyperswitch PR.
GooglePaySessionResponse.connector, GooglePayThirdPartySdk.connector and
PaypalClientAuthenticationResponse.connector are still string and have the same
problem — happy to follow up on those separately if you want them moved too.

Motivation and Context

Follow-up to review feedback on juspay/hyperswitch#13481, which changes the same field
on hyperswitch's ApplepaySessionTokenResponse from String to hyperswitch's Connector
enum. Because UCS sends the field as a string, that PR currently has to parse it at the
boundary with Connector::from_str. @hrithikesh026 pointed out that this couples
hyperswitch to UCS's serialization format, and @Nithin1506200 asked for the enum change to
be made here so the conversion becomes explicit on both sides.

Ordering: this lands and gets tagged first, then hyperswitch#13481 bumps
unified-connector-service-client and reads the enum directly.

Additional Changes

• This PR modifies the API contract
• This PR modifies application configuration/environment variables

This is a wire-breaking change on field 3 (string is length-delimited, an enum is a
varint), so buf breaking will fail under the FILE ruleset. Per the guidance the
proto-checks job prints, that means the PR needs the proto-breaking-approved label — I
can't add it myself.

Heads up on a conflict between two of your workflows, which cost me a red check here: the
Fail on unapproved breaking change step in proto-checks.yml tells you to "use proto!:
as the PR title type", but Verify PR title follows conventional commit standards rejects
it with Commit type `proto` not allowed. That job runs on pull_request_target with no
checkout step (deliberately, per the comment at the top of the file), so cog.toml — where
proto is registered as a commit type — is never on disk, and cocogitto falls back to its
built-in types. Every merged proto change I looked at uses the scope form instead
(feat(proto):, fix(proto):), so I've retitled this one refactor(proto)!:. Either the
proto-checks message or the title job's config probably wants a fix.

The one checklist item that isn't satisfied yet is "migration PR is already merged and
deployed": hyperswitch#13481 is open, and pins UCS by git tag, so it won't see this change
until it bumps the tag. That's the sequence @Nithin1506200 proposed on that PR ("we will
get it merged and you can bump it here later"), but if you'd rather not break the wire at
all, the alternative is the additive route the same job suggests — add
Connector connector_type = 9;, mark field 3 [deprecated = true], and drop it in a
follow-up once consumers have moved. Say the word and I'll rework it that way.

Also worth flagging, found while checking the mapping is total: ConnectorEnum::RazorpayV2
and ConnectorEnum::Affirm have no counterpart in the proto Connector enum (there is
RAZORPAY but no RAZORPAY_V2, and no AFFIRM at all). Today
grpc_connector_from_connector_enum quietly turns both into CONNECTOR_UNSPECIFIED.
Neither serves an Apple Pay session, so nothing here regresses, but it's a pre-existing gap
worth a separate look.

How did you test it?

Added apple_pay_session_connectors_map_to_their_proto_variants in
domain_types/src/types.rs, covering the only two connectors that build an Apple Pay
session token — it asserts ConnectorEnum::Braintree and ConnectorEnum::Trustpay land on
Connector::Braintree and Connector::Trustpay, which is what breaks if either side is
renamed.

On the hyperswitch side, hyperswitch#13481 has a matching test pinning "braintree" and
"trustpay" as the exact strings UCS sends today.

I wasn't able to run the full cargo clippy / cargo nextest matrix locally — this
workspace needs more memory than my machine has — so I'm relying on CI for those and will
turn around anything that comes back red. cargo +nightly fmt --check is clean on every
file I touched.

[Copilot]

Pull request overview

Updates the Apple Pay client-auth/session-token payload to carry the connector as a protobuf Connector enum instead of a free-form string, aligning the gRPC contract with existing enum usage and making connector mapping explicit at conversion boundaries.

Changes:

• Changed ApplepayClientAuthenticationResponse.connector in payment.proto from string to Connector (enum).
• Updated domain model + gRPC conversion to use ConnectorEnum and added a ForeignTryFrom<ConnectorEnum> mapping to the proto enum (with a unit test).
• Updated Braintree and Trustpay Apple Pay session-token builders to pass ConnectorEnum::{Braintree, Trustpay}.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
crates/types-traits/grpc-api-types/proto/payment.proto	Wire-contract change: Apple Pay connector field is now a proto enum.
crates/types-traits/domain_types/src/types.rs	Converts ConnectorEnum to proto Connector during Apple Pay gRPC response building; adds test coverage.
crates/types-traits/domain_types/src/connector_types.rs	Domain Apple Pay response now stores connector as ConnectorEnum.
crates/integrations/connector-integration/src/connectors/trustpay/transformers.rs	Uses ConnectorEnum::Trustpay when constructing Apple Pay session token data.
crates/integrations/connector-integration/src/connectors/braintree/transformers.rs	Uses ConnectorEnum::Braintree when constructing Apple Pay session token data.

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