fix(toolsets): send a return description the way pydantic-ai sends one

[DEENUU1]

render appended a prose Returns: … paragraph. pydantic-ai does something else with a docstring's Returns: section — it wraps the prose in <summary> and the return description in <returns>:

<summary>Draw a chart of numbers you already have, so the user can see them.

Use this whenever the user asks to plot, chart, graph or compare figures …</summary>
<returns>
<description>The chart specification, already on its way to the user.</description>
</returns>

So a host that registers these tools beside tools of its own ends up with two conventions in one tool list: create_chart, built from a docstring, arriving as XML, and ls arriving as prose describing the same kind of thing. That shape was invented here, and inventing it was the mistake — the framework already had one.

render emits the framework's shape now.

The test pins it against a tool pydantic-ai renders itself rather than against a string literal:

@native.tool_plain
def demo() -> str:
    """Do a thing.

    A second paragraph of usage.

    Returns:
        One line per entry.
    """

assert ToolText(summary=..., usage=..., returns=...).render() == native.tools["demo"].tool_def.description

If pydantic-ai changes how it renders that section, this fails here rather than leaving these tools quietly speaking the old dialect.

A ToolText with no returns is still plain prose, which is also what the framework does with a docstring that has no Returns: section.

The *_DESCRIPTION constants carry the new shape with them. A catalogue that wanted the prose reads ToolText.summary — the sentence it wanted in the first place, and what agenticos already shows beside its approval checkboxes.

Verified: 1716 tests pass, coverage 100%, pyright and mypy strict clean.