Forward resource-read _meta in vMCP and the transport bridge

[jhrozek]

Part of #6027. Blocked on stacklok/toolhive-core#194 landing and being released.

Work

Once mcpcompat exposes a result-returning resource handler, consume it at the
two sites that currently discard backend resource _meta:

1. vMCP Serve path — pkg/vmcp/server/serve_handlers.go:
   coreResourceHandler and coreResourceTemplateHandler return a bare
   []mcp.ResourceContents and drop result.Meta. Route it through
   conversion.ToMCPMeta(result.Meta) so it gets the same reserved-key strip
   every other Legacy egress already gets (Strip reserved io.modelcontextprotocol/* keys from backend response _meta (shared Legacy+Modern helper) #5986 / Strip reserved io.modelcontextprotocol/* keys from responses #6024).

2. Transport bridge — pkg/transport/bridge.go:226,241, same shape, same
   loss.

The client side needs no change: vmcp.ResourceReadResult.Meta is already
populated on both the Legacy and Modern paths
(pkg/vmcp/client/client.go legacyReadResource / modernReadResource).

Also fix the stale comment

pkg/vmcp/client/meta_integration_test.go:217 says resource _meta is
impossible "Due to MCP SDK constraints". That is wrong — go-sdk supports it;
the gap was mcpcompat's mcp-go-shaped handler signature. Update the comment and
turn the test into a real preservation assertion.

Why it matters

Closes the Legacy/Modern fork introduced by #6024: Modern preserves-and-strips
resource _meta (its hand-rolled envelope bypasses the SDK handler), Legacy
drops it. Same backend, same request, different client-visible metadata
depending on the negotiated revision.

Also finishes the resource half of #2640, which shipped tools/call and
prompts/get and closed with this item open.

Acceptance

• A backend's non-reserved resource-read _meta (trace ids, custom fields)
  reaches the client on both revisions.
• Reserved io.modelcontextprotocol/* keys are still stripped on both, per Strip reserved io.modelcontextprotocol/* keys from backend response _meta (shared Legacy+Modern helper) #5986.
• TestMetaPreservation_ReadResource asserts preservation instead of
  documenting a limitation.

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