session: CreateRequest missing DisplayName field (parity with Python ADK)

[rubda]

Is your feature request related to a specific problem?

The Python ADK's VertexAiSessionService.create_session accepts **kwargs that are forwarded to the Vertex AI API, allowing callers to set display_name on session creation. The Go ADK's session.CreateRequest struct has no equivalent field, so there is no way to set a human-readable name for sessions at creation time.

This forces workarounds like storing names in session state or making separate UpdateSession API calls after creation.

Proposed Solution

Add a DisplayName string field to session.CreateRequest and pass it through to the Vertex AI protobuf Session.DisplayName in vertexai_client.go.

The field is optional and zero-valued by default, so it is fully backward compatible. Non-Vertex AI backends (in-memory, database) can safely ignore it.

Impact on your work

We use ADK sessions for a chat assistant and need user-facing session names derived from the first message. Without DisplayName support, we store names in session state and read them back in our application layer — unnecessary complexity for what should be a simple field passthrough.

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Alternatives Considered

1. Store name in session state: Works, but the name lives in app-level state rather than the native DisplayName field. Requires custom toSession logic to extract it.
2. Call UpdateSession after creation: Adds an extra API round-trip per session creation for a field that could be set at creation time.

Willingness to contribute

Yes — I have a branch with the implementation and tests ready to submit as a PR.

Proposed API / Implementation

// session/service.go
type CreateRequest struct {
    AppName     string
    UserID      string
    SessionID   string
    State       map[string]any
    // DisplayName is a human-readable name for the session.
    // Optional: only used by backends that support it (e.g. Vertex AI).
    DisplayName string
}

// session/vertexai/vertexai_client.go — createSession
pbSession := &aiplatformpb.Session{
    UserId:      req.UserID,
    DisplayName: req.DisplayName, // new
}

Additional Context

The Python ADK supports this via **kwargs passthrough (source):

async def create_session(self, *, app_name, user_id, state=None, session_id=None, **kwargs):
    # kwargs like display_name are forwarded to the Vertex AI API

The underlying Vertex AI protobuf Session message already has DisplayName — the Go ADK just doesn't expose it through CreateRequest.