feat(backend): expose v2beta1 auth and visualization via HTTP gateway. Part of #13283

[Amr-Shams]

Description of your changes:

This PR adds v2beta1 parity for Auth and Visualization exposure.

• Registers AuthService and VisualizationService for v2beta1 on the HTTP gateway (startHTTPProxy), so REST paths are reachable.
• Keeps existing v1beta1 behavior unchanged.

Issue: 13283

Validation:

• go test -v ./backend/src/apiserver/server
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[google-oss-prow]

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[google-oss-prow]

Hi @Amr-Shams. Thanks for your PR.

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[Copilot]

Pull request overview

This PR aims to provide v2beta1 parity for Auth and Visualization by registering their v2beta1 handlers/servers so the REST paths are reachable via the gRPC-Gateway, while preserving existing v1beta1 behavior.

Changes:

• Register v2beta1 Auth and Visualization services on both the gRPC server and HTTP gateway.
• Split Auth/Visualization implementations into explicit V1 vs V2 server types.
• Expand/adjust unit tests to cover v1beta1 and v2beta1 flows for visualization and auth.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
backend/src/apiserver/server/visualization_server.go	Refactors visualization server into V1/V2 variants and centralizes URL/aliveness logic.
backend/src/apiserver/server/visualization_server_test.go	Adds V1/V2 test coverage and new helpers for visualization URL + service interaction.
backend/src/apiserver/server/auth_server.go	Splits Auth server into V1/V2 types and adds V1 request validation helper.
backend/src/apiserver/server/auth_server_test.go	Adds v2beta1 auth tests and updates V1 tests to use the V1 server type.
backend/src/apiserver/main.go	Registers V2 Auth/Visualization with gRPC and HTTP gateway; updates V1 registration to use new constructors.