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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.77%. Comparing base (acc6df3) to head (eaad88c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6119      +/-   ##
==========================================
+ Coverage   72.69%   72.77%   +0.08%     
==========================================
  Files         741      741              
  Lines       77212    77147      -65     
==========================================
+ Hits        56127    56142      +15     
+ Misses      17127    17050      -77     
+ Partials     3958     3955       -3     

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[ChrisJBurns]

Two stale doc comments elsewhere in this file describe the removed decorator path — not required for this PR, but worth a quick follow-up.

[ChrisJBurns]

New()'s doc comment at lines 95-96 still says: "It builds the decorating session factory from cfg, wiring the optimizer and composite tool layers internally." That's no longer accurate — the factory is now just cfg.Base with no decorators, and the optimizer is surfaced solely via sm.optimizerFactory/OptimizerFactory() for the Serve layer to consume. Worth rewording.

[ChrisJBurns]

The comment at lines 132-133 above sm := &Manager{...} says "Build the Manager first so we can reference sm.Terminate and sm.sessions directly in closures..." — with buildDecoratingFactory (and its sm.Terminate param) gone, nothing built in New() references sm.Terminate anymore, only sm.loadSession/sm.checkSession/sm.sessions. The comment should drop the sm.Terminate mention.