feat(session): add DisplayName field to CreateRequest

[rubda]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: session: CreateRequest missing DisplayName field (parity with Python ADK) #706

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Added:

• TestCreateSessionProto_DisplayName in session/vertexai/vertexai_test.go — verifies DisplayName is mapped to the protobuf Session.
• Create/with_display_name in the shared service test suite (session/session_test/service_suite.go) — verifies all implementations accept DisplayName without error.

=== RUN   TestCreateSessionProto_DisplayName
=== RUN   TestCreateSessionProto_DisplayName/display_name_is_set_on_proto
=== RUN   TestCreateSessionProto_DisplayName/empty_display_name_results_in_empty_proto_field
--- PASS: TestCreateSessionProto_DisplayName (0.00s)

=== RUN   Test_inMemoryService/Create/with_display_name
--- PASS: Test_inMemoryService/Create/with_display_name (0.00s)

Manual End-to-End (E2E) Tests:

The change adds one field to CreateRequest and passes it through to aiplatformpb.Session.DisplayName when creating Vertex AI sessions. Verified by inspecting the proto construction and running the existing replay-based Vertex AI test suite — all existing tests continue to pass since DisplayName defaults to the zero value (empty string), which is the previous behavior.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

The Python ADK supports this via **kwargs passthrough in VertexAiSessionService.create_session. The underlying Vertex AI protobuf Session message already has DisplayName — this change simply exposes it through the Go ADK's CreateRequest.

Changes:

• session/service.go — Add DisplayName string to CreateRequest
• session/vertexai/vertexai_client.go — Set DisplayName on the protobuf Session
• session/vertexai/vertexai_test.go — Proto construction test
• session/session_test/service_suite.go — Shared suite test