feat(memory): add AddEventsToMemory for incremental ingestion

[mdabydeen]

Link to Issue or Description of Change

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

• Closes: memory.Service is missing AddEventsToMemory (incremental ingestion, parity with adk-python) #1160

2. Problem / Solution:

Problem: adk-python's BaseMemoryService.add_events_to_memory(*, app_name, user_id, events, session_id=None, custom_metadata=None) lets callers persist a subset of events (e.g. just the latest turn) as an incremental delta, instead of re-ingesting the whole session via add_session_to_memory. memory.Service in adk-go has no equivalent — AddSessionToMemory is the only ingestion path, and it wholesale-replaces the stored events for a session ID on every call (no per-event dedup at all).

Solution:

• Add AddEventsToMemory(ctx, *AddEventsToMemoryRequest) error to memory.Service, with fields mirroring Python's kwargs (AppName, UserID, Events, optional SessionID, optional CustomMetadata).
• Implement in memory/inmemory.go: extracted the event→value conversion (skip events with no text content) into a shared eventToValue helper reused by both AddSessionToMemory and the new method, and dedup incoming events by event.ID against whatever's already stored for that (app, user, session) scope — mirroring in_memory_memory_service.py's dedup behavior, so repeated calls with overlapping events are idempotent.
• memory/vertexai/vertexai.go returns an explicit "not supported" error for now. adk-python's Vertex AI Memory Bank backend does implement this against the real ingestion API, but porting that is a distinctly larger, separate effort from this interface addition — happy to follow up separately if useful.

Testing Plan

Unit Tests:

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

Added Test_inMemoryService_AddEventsToMemory covering: events become searchable, repeated calls with overlapping events are deduped by ID, isolation across sessions/users, and that contentless events are ignored (matching existing AddSessionToMemory semantics). Added a standalone unit test asserting the Vertex AI backend's explicit unsupported error.

$ go test -race -mod=readonly -count=1 -shuffle=on ./...
ok  	google.golang.org/adk/v2/memory	...
ok  	google.golang.org/adk/v2/memory/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 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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