build(deps): require subagents-pydantic-ai 0.2.18 (#199)

## What this does

Adopts **subagents-pydantic-ai 0.2.18**
([subagents#78](vstorm-co/subagents-pydantic-ai#78)).

0.2.18 removes the implicit `default_model="openai:gpt-4.1"` from
`create_subagent_toolset`: there is now no library-chosen default, and a
toolset
that leaves the general-purpose delegate on (the default) with no model
raises at
construction rather than building that delegate on whatever provider
credential
the process environment happens to hold.

## What changed

- `pyproject.toml` — `subagents-pydantic-ai` bumped `>=0.2.12` →
`>=0.2.18`.
- `tests/test_toolsets_extended.py` —
`test_create_with_custom_subagents` built
`create_subagent_toolset(subagents=[...])` with no model and relied on
the old
default, so it began raising under 0.2.18. It now passes
`default_model="test"`,
building the general-purpose delegate on pydantic-ai's `TestModel` like
the rest
  of the suite — no live model, offline as before.

**Product code needs no change.** `pydantic_deep/agent.py` already
passes
`default_model` and `include_general_purpose=False` to
`create_subagent_toolset`,
and `features/teams/toolset.py` calls `_compile_subagent(config,
member.model)`
with a real model. The only thing that relied on the old implicit
default was the
one test above.

## Testing

- Full suite: **2785 passing, 100% coverage** (the gate).
- `ruff format --check` and `ruff check`: clean.

Author: DEENUU1

pyproject.toml

@@ -44,7 +44,7 @@ dependencies = [
     "pydantic-ai-todo>=0.2.7",
     "pydantic-ai-backend[console]>=0.2.18",
     "summarization-pydantic-ai>=0.1.11",
-    "subagents-pydantic-ai>=0.2.12",
+    "subagents-pydantic-ai>=0.2.18",
     "pydantic-ai-shields>=0.3.4",
     "pydantic>=2.0",
     "chardet>=5.0.0",

tests/test_toolsets_extended.py

@@ -35,7 +35,7 @@ def test_create_with_custom_subagents(self):
                 instructions="You research topics thoroughly.",
             ),
         ]
-        toolset = create_subagent_toolset(subagents=subagents)
+        toolset = create_subagent_toolset(default_model="test", subagents=subagents)
         assert toolset is not None
 
     def test_create_without_general_purpose(self):