test(session): add concurrent regression test for State.All()

[nuthalapativarun]

Link to Issue

Fixes #561

Description of Change

Problem:
state.All() previously released the read lock between each map iteration step (s.mu.RUnlock() inside the range s.state loop), allowing concurrent Set() calls to mutate the map mid-iteration and trigger a fatal "concurrent map iteration and map write" panic.

Solution:
The fix (snapshotting the map under lock via maps.Clone before iterating) is already in session/inmemory.go. However, no concurrent test existed to verify this and guard against future regressions. This PR adds TestInMemoryState_All_ConcurrentSet which:

• Creates a session with initial state
• Races State.All() (200 iterations) against 8 goroutines each appending events with state deltas concurrently
• Passes cleanly under go test -race

Testing Plan

Unit Tests:

• TestInMemoryState_All_ConcurrentSet added to session/inmemory_test.go
• All tests pass with the race detector enabled

$ go test -race ./session/...
ok  	google.golang.org/adk/session	1.357s
ok  	google.golang.org/adk/session/database	1.619s
ok  	google.golang.org/adk/session/vertexai	35.586s

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.

[nuthalapativarun]

Hi, just checking in on this one — happy to make any adjustments if something needs to change. Let me know if there's anything blocking review. Thanks!