Triage vMCP conformance gaps: content types, templates, server-to-client forwarding

Pointing the MCP conformance suite at a vMCP endpoint (#5844 item 2, PR #5852) surfaced failing scenarios. Some are clear aggregator limitations (sampling, completions, resource subscribe/unsubscribe) — vMCP terminates the protocol and re-serves an aggregated view, and does not proxy these. Others are less obviously "expected" and may be real gaps/bugs in the migrated mcpcompat/go-sdk serving surface, worth investigating individually the way prompts/get was (that one turned out to be a real bug, fixed in #5857 via toolhive-core SessionWithPrompts).

Baselined in expected-failures-vmcp.yaml for now so the conformance job is green, but tracked here:

Likely worth a real fix

• tools-call-mixed-content and tools-call-embedded-resource — tool results carrying mixed / embedded-resource content. Basic tools-call passes, so this is a content-type conversion gap in the vMCP tool-result path, not a fundamental limitation.
• resources-templates-read — resource templates. Basic resources-list / resources-read-text / -binary pass, so template handling specifically is missing.

Server→client forwarding (may be architectural, confirm)

• tools-call-with-progress, tools-call-with-logging — the server sends progress/logging notifications to the client mid-call; vMCP would need to forward them.
• tools-call-elicitation, elicitation-sep1034-defaults, elicitation-sep1330-enums — server→client elicitation.

Accepted aggregator limitations (no action expected)

• tools-call-sampling, completion-complete, resources-subscribe, resources-unsubscribe.

Each investigation should follow the prompts model: reproduce via the conformance artifact's expected-vs-actual, root-cause in the vMCP serve path, and if it's a real bug fix it and drop it from the baseline.

Context: PR #5852, part of epic #5743.

Start with expected-failures-vmcp.yaml and reproduce one listed scenario using the conformance artifact's expected-versus-actual output. Trace the failure through the vMCP serve path, using the prompts investigation as a model, and determine whether it is an aggregator limitation or a real bug. Done means fixing confirmed bugs and removing them from the baseline.