feat(express): Automatically suppress non-renderable UI objects

[gadenbuie]

For #1771

Example app:

import duckdb
import pandas as pd

from shiny.express import input, render, ui

# Create a sample DataFrame
df = pd.DataFrame(
    {
        "id": range(1, 6),
        "name": ["Alice", "Bob", "Charlie", "David", "Eve"],
        "age": [25, 30, 35, 28, 22],
    }
)


# Register DataFrame with DuckDB
con = duckdb.connect(":memory:")
con.register("people", df)

ui.input_select("person", "Choose a person:", choices=df["name"].tolist())


@render.text()
def value():
    res = con.execute("SELECT age FROM people WHERE name = ?", [input.person()])
    return f"You choose: {input.person()} who is {res.fetchone()[0]} years old."

As mentioned in #1771 the above example app would fail to launch in Shiny Express due to the con.register() line. This PR makes is so that lines with un-renderable objects are not introduced in the UI.

It gives a warning message like the following, which we could wordsmith as needed. (Currently a print() call but we might be able to use warnings.warn() to include more context.)

Express has suppressed the object `<duckdb.duckdb.DuckDBPyConnection object at 0x126107a70>`
because it is of type <class 'duckdb.duckdb.DuckDBPyConnection'>. Coerce to HTML, a string,
or an htmltools tag to display this object.