chore(a2a): migrate from adka2a to adka2a/v2

[aheritier]

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Background

google.golang.org/adk v1.6.0 (upgraded in #4015) deprecated google.golang.org/adk/server/adka2a in favour of google.golang.org/adk/server/adka2a/v2. The v0 compatibility shim was kept in #4015 with a narrow SA1019 suppression in .golangci.yml to keep the upgrade self-contained.

What needs to change

Migrating to adka2a/v2 is not a mechanical find-and-replace. The v2 executor contract is different:

	adka2a (v0, current)	adka2a/v2 (target)
Executor signature	Execute(ctx, *a2asrv.RequestContext, eventqueue.Queue) error	Execute(ctx, *a2asrv.ExecutorContext) iter.Seq2[a2a.Event, error]
A2A dependency	github.com/a2aproject/a2a-go (v0)	github.com/a2aproject/a2a-go/v2

This requires a deliberate rewrite of:

• pkg/a2a/executor_wrapper.go — replace fixingQueue + eventqueue.Queue wiring with the new iter.Seq2 streaming contract
• pkg/a2a/server.go — update adka2a imports and any RequestContext / runner wiring
• Tests: pkg/a2a/executor_wrapper_test.go, pkg/a2a/server_test.go, e2e/a2a_test.go

Once migration is complete, remove the SA1019 exclude-rule added to .golangci.yml in #4015.

Acceptance criteria

• google.golang.org/adk/server/adka2a/v2 used in place of the deprecated shim
• github.com/a2aproject/a2a-go/v2 replaces v0 where required by adka2a/v2
• SA1019 exclude-rule for adka2a removed from .golangci.yml
• task lint, task test, and A2A e2e tests pass