fix: pin pydantic-ai to <2 so the package imports on a fresh install

[OfficialAbhinavSingh]

Problem

On a fresh clone, uv sync + uv run pytest fails at collection:

ImportError: cannot import name 'OpenAIModel' from 'pydantic_ai.models.openai'
  src/dlens/__init__.py -> .agents -> ._models
  src/dlens/agents/_models.py:2: from pydantic_ai.models.openai import OpenAIModel

Root cause: pyproject.toml pins pydantic-ai>=1.48.0 with no upper bound, so uv resolves pydantic-ai 2.x (currently 2.12.0). In the 2.x line, OpenAIModel was removed from pydantic_ai.models.openai (split into OpenAIChatModel / OpenAIResponsesModel). The import dlens chain hits _models.py:2 and the whole package — and therefore the smoke test and every script — fails to import.

Reproduce

git clone https://github.com/ML4SCI/DeepLense-AI-Scientist.git
cd DeepLense-AI-Scientist
uv sync
uv run pytest        # ImportError at collection

Fix

Pin to the 1.x line the current code targets (the OpenAIModel wrapper at _models.py:12 and the import at :2 use the 1.x API):

-    "pydantic-ai>=1.48.0",
+    "pydantic-ai>=1.48.0,<2",

Verification

After the pin, a fresh uv sync resolves pydantic-ai 1.107.1 and:

$ uv run pytest tests/
tests/test_smoke.py::test_ping PASSED                              [100%]
1 passed
$ uv run python -c "import dlens; from dlens.agents import OllamaModel, DLensBaseAgent; print('import ok')"
import ok

Note

Migrating to pydantic-ai 2.x (OpenAIChatModel / OpenAIResponsesModel, updating the OpenAIModel wrapper) is a larger, separate change — deliberately left out here to keep this a minimal "repo builds on a fresh clone again" fix. Happy to follow up with the 2.x migration if that's the preferred direction.