Namespace transform does not rewrite ResourceLink URIs returned from proxied tools #4154

What happened?

summary

With a Namespace transform on a proxy, listed resources are namespaced
correctly.
However, ResourceLink.uri values returned from a proxied
tools/call result keep the upstream URI instead of the proxied URI.

Tested on FastMCP 3.3.0.

minimal example

from fastmcp.server import create_proxy
from fastmcp.server.transforms import Namespace

proxy = create_proxy(upstream_client, name="Proxy-everything")
proxy.add_transform(Namespace("everything"))

An upstream resource looking like this:

demo://resource/dynamic/text/2

will be exposed correctly as:

demo://everything/resource/dynamic/text/2

But if an upstream tool returns:

mcp.types.ResourceLink(
    type="resource_link",
    name="dynamic-text",
    uri="demo://resource/dynamic/text/2",
)

the proxied tool result still contains:

demo://resource/dynamic/text/2

Observed Behavior

resources/list:
  upstream: demo://resource/dynamic/text/2
  proxied:  demo://everything/resource/dynamic/text/2

resources/read:
  demo://everything/resource/dynamic/text/2
  -> works

tools/call result content:
  ResourceLink.uri == demo://resource/dynamic/text/2
  -> raw upstream URI leaks through unchanged

expected behavior

ResourceLink content returned by proxied tools should expose the same proxied
URI as resources/list and resources/read.

scope of this issue

This is specifically about mcp.types.ResourceLink content inside tools/call
results. Normal resources/list, resource_templates/list, and
resources/read already behave correctly through the proxy.

Session-scoped resources returned by some tools may have additional lifecycle
constraints beyond URI rewriting. This issue is about ordinary ResourceLink URI
projection only.

Example Code

Version Information

FastMCP version:                                                                              3.3.0
MCP version:                                                                                 1.27.0
Python version:                                                                              3.14.3
Platform: Windows-11-10.0.26200-SP0
FastMCP root path: its a uv venv trust