Pin forwarding tools/call against a stale-Legacy stateless backend

[JAORMX]

Summary

Part of #5992. That issue's headline problem — a Legacy-cached backend redeployed stateless/Modern staying pinned Legacy forever — was fixed in #5997 (legacyInit reads the genuinely-negotiated protocol version off every InitializeResult and flips the cache in-band, pinned by TestListCapabilities_MisCachedLegacy_SelfHealsViaSDKNegotiation). #6002 re-scoped the TTL re-probe as an open question, not requested work.

What remained from the issue body is the "also worth a test" half:

The "stale Legacy cache is harmless" property is currently pinned only for ListCapabilities (forwarding=false). The forwarding CallTool path (WithContinuousListening standalone GET stream) against a real stateless Modern backend is untested — low risk, but the harmlessness claim is asserted more broadly than it's covered.

This adds that test.

Why the forwarding arm is the one worth pinning. Binding forwarders makes newStreamableHTTPClient enable transport.WithContinuousListening(), which opens the standalone server→client GET stream. Opened against a session-less backend, that stream is exactly what made the pooled session-factory connect fail in #6006 (subscriptions/listen → "session not found"). The per-call legacy path survives because go-sdk's streamableClientConn.sessionUpdated opens that stream only when the negotiated protocol is < 2026-07-28 (mcp/streamable.go:2096) — and the SDK's Modern-first client negotiates Modern on the mis-cached-Legacy hop, so the gate suppresses the stream entirely. legacyInit then flips the cache to Modern in-band.

A regression in that version gate (or a shim change opening the stream unconditionally) would break only this forwarding path — invisible to the existing ListCapabilities pin. This test makes it fail loudly.

Type of change

• Other (describe): test-only

Test plan

• Unit tests (task test) — full pkg/vmcp/client package green with -race
• Linting (task lint-fix)

TestCallTool_MisCachedLegacy_ForwardingAgainstStatelessBackend: binds stub forwarders, mis-caches Legacy against a real stateless (Modern) go-sdk backend, drives the forwarding CallTool, and asserts both the successful round-trip and the in-band cache flip to Modern. The harness dependency (factory Connect + legacyInit) is the exact path #6006 proved fatal without the gate, so the pin is non-vacuous.

API Compatibility

• This PR does not break the v1beta1 API.

No production code changed.

Does this introduce a user-facing change?

No.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.08%. Comparing base (a6a0d4c) to head (079ddc0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6010      +/-   ##
==========================================
- Coverage   72.15%   72.08%   -0.07%     
==========================================
  Files         720      720              
  Lines       74744    74744              
==========================================
- Hits        53929    53879      -50     
- Misses      16942    17010      +68     
+ Partials     3873     3855      -18     

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