feat(session): add GetUserState for parity with adk-python

[mdabydeen]

Link to Issue or Description of Change

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

• Closes: session.Service is missing GetUserState (parity with adk-python) #1158

2. Problem / Solution:

Problem: adk-python's BaseSessionService exposes get_user_state(app_name, user_id) to read user-scoped state without requiring an active session — useful for bootstrapping context before create_session without an expensive list_sessions + get_session round trip. session.Service in adk-go has no equivalent, even though the in-memory backend already stores this data internally (inMemoryService.userState) and the database backend already has a helper (fetchStorageUserState) that could back it directly.

Solution:

• Add GetUserState(ctx, *GetUserStateRequest) (*GetUserStateResponse, error) to the session.Service interface.
• Implement it in session/inmemory.go using the existing userState map.
• Implement it in session/database/service.go using the existing fetchStorageUserState helper.
• In session/vertexai/vertexai.go, return an explicit "not supported" error, mirroring adk-python's VertexAiSessionService.get_user_state, which also doesn't support this.
• Update the FakeSessionService test double in server/adkrest/internal/fakes to satisfy the interface.

Testing Plan

Unit Tests:

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

Added a shared GetUserState battery to session/sessiontestsuite, gated behind a new SuiteOptions.SupportsGetUserState flag, covering: empty state for unknown users, reading un-prefixed keys set both at session-create time and via a later AppendEvent state delta, and per-user isolation. Opted in for the in-memory and database backends (both of which now pass it). Added a standalone unit test asserting the Vertex AI backend returns an error, since that backend's contract-test run is driven by recorded gRPC fixtures I can't regenerate without live GCP credentials.

$ go test -race -mod=readonly -count=1 -shuffle=on ./...
ok  	google.golang.org/adk/v2/session	...
ok  	google.golang.org/adk/v2/session/database	...
ok  	google.golang.org/adk/v2/session/vertexai	...
... (all other packages ok)

go build, go vet, and go mod tidy -diff are all clean.

Manual End-to-End (E2E) Tests:

N/A — internal service API addition, covered by unit/contract tests above.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• 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. (N/A, see above)
• Any dependent changes have been merged and published in downstream modules. (N/A)

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