fix(mcp): name the offending path in computeURIFilepath localize error

[PratikDhanave]

computeURIFilepath formats filepath.Localize's error-path return value:

uriFilepathRel, err := filepath.Localize(strings.TrimPrefix(uri.Path, "/"))
if err != nil {
    return "", fmt.Errorf("%q cannot be localized: %w", uriFilepathRel, err)
}

On error, filepath.Localize returns "", so the message is always "" cannot be localized: ... and never names the path that failed — unhelpful precisely on the path-traversal rejection path where you want to see the offending input. Format the input relative path instead.

Added a regression test asserting the offending path appears in the error (fails before the fix: "" cannot be localized: invalid path; passes after). go build ./..., go test ./mcp/ pass.