fix(ci): add langchain-core to [dev] so test collection passes (#12)

The SDK's import chain (nullrun.__init__ -> nullrun.decorators ->
nullrun.instrumentation.langgraph -> 'from langchain_core.callbacks
import BaseCallbackHandler') runs at pytest *collection* time, not at a
specific test. With CI installing [dev] only, every test in the suite
errored on collection with:

    ModuleNotFoundError: No module named 'langchain_core'

This is the same class of bug that 'nullrun[langgraph]' exists to
prevent for end users, except the dev install never benefited from
the extras indirection.

Fix: add 'langchain-core>=0.3,<1.0' to the [dev] extras. The
heavier 'langgraph' / 'langchain' extras pull in stacks the unit
tests don't use; the bare core is the smallest dep that makes the
import chain resolve and unblocks test collection on every
supported Python (3.10 / 3.11 / 3.12) on every PR.

Validation: locally on Python 3.14.2 (which is outside the
3.10/3.11/3.12 matrix that CI tests), 'pip install -e .[dev]'
followed by 'pytest tests/' runs 443/443 + 9/9 new byte-mismatch
unit tests, no collection error. CI will re-confirm on the 3.10 /
3.11 / 3.12 matrix.

Author: maltsev-dev