Remove legacy optimizer decorator path

[kocaemre]

Summary

• The optimizer-without-AdvertiseFromCore path is now rejected by sessionmanager.New, so the old factory-level optimizer decorator branch can no longer run.
• Remove the unreachable legacy optimizer decorator path and its private tool-adaptation helper.
• Wire the session manager directly to the undecorated base factory; the resolved optimizer factory continues to be surfaced to the Serve layer via Manager.OptimizerFactory().

Fixes #6103

Type of change

• Bug fix
• New feature
• Refactoring (no behavior change)
• Dependency update
• Documentation
• Other (describe):

Test plan

• Unit tests (task test)
• E2E tests (task test-e2e)
• Linting (task lint-fix)
• Manual testing (describe below)

Manual testing:

• PATH=/usr/local/go/bin:/root/go/bin:$PATH go test ./pkg/vmcp/server/sessionmanager
• PATH=/usr/local/go/bin:/root/go/bin:$PATH go test ./pkg/vmcp/server/sessionmanager ./pkg/vmcp/server
• PATH=/usr/local/go/bin:/root/go/bin:$PATH go test -race ./pkg/vmcp/server/sessionmanager
• PATH=/usr/local/go/bin:/root/go/bin:$PATH task lint
• git diff --check

API Compatibility

• This PR does not break the v1beta1 API, OR the api-break-allowed label is applied and the migration guidance is described above.

Does this introduce a user-facing change?

No.

Special notes for reviewers

This is intentionally a deletion-only follow-up to the constructor guard merged in #6033. The optimizer is still resolved, cleaned up by the manager, and consumed by the Serve layer when enabled; the removed path was unreachable because New rejects an optimizer unless AdvertiseFromCore is set.