ci: verify adapter imports on a bare install

[yash161004]

Closes #6.

Problem

The test job installs .[dev], and dev pulls openeval-core[fixtura,langchain,openai]. Every optional dependency is therefore always present in CI, so the suite cannot detect an adapter that has started requiring one.

That is precisely how the bug fixed in #5 shipped. openeval/adapters/__init__.py eagerly imported the LangChain adapter, so pip install openeval-core could not import any adapter — while every test stayed green. #5 removed the eager import but added nothing to stop it returning.

Change

A bare-install job that reproduces what a real user gets (pip install ., no extras) and asserts four things:

Assertion	Why
fixtura + openai adapters import	They have no third-party dependency and must never gain one implicitly
LangChain adapter fails only on access, and cleanly	Its dep is genuinely optional; lazy resolution must keep it that way
Unknown attribute still raises AttributeError	The lazy __getattr__ must not swallow typos
The optional deps really are absent	Otherwise the job passes vacuously if an adapter dep ever becomes a base dependency — which would "fix" a failure here the wrong way

Wired into all-tests-passed, the required check, so it actually gates rather than sitting decorative.

One assumption corrected while writing it: from_openai_messages imports fine with no extras. That adapter parses message dicts and never imports the openai package — openai is in the extras for tests and examples only. The guard asserts that reality rather than a symmetry that doesn't exist.

Verified in a clean venv

confirmed: no optional adapter dependencies present
all import OK
expected on access: No module named 'langchain_core'
expected: module 'openeval.adapters' has no attribute 'does_not_exist'

And — the part that matters — reinstating the old eager import makes it fail:

import openeval.adapters.fixtura
  File ".../openeval/adapters/__init__.py", line 1, in <module>
    from .langchain import from_langchain_run
ModuleNotFoundError: No module named 'langchain_core'
>>> guard correctly FAILS - regression caught

Restoring the lazy version makes it pass again. A guard that can't catch the regression it exists for is worse than none, so this was checked rather than assumed.

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