tests: Move langchain under toxgen

[sentrivana]

• move the langchain test suites to be governed by toxgen
  • this indirectly results in removing the -latest tests in the AI group 🎉
    • -latest tests predate toxgen and langchain was the last non-toxgen test suite (the rest was just skipped) -- all AI tests are now pinned
• updated the naming scheme to use dashes instead of underscores for variants so that it's clearer if something is part of the name of the integration or if it denotes a variant
  • for instance, openai-base means this is the base variant of the openai test suite, but openai_agents means this is the openai_agents test suite (no variant)

I'm explicitly ignoring the two alpha versions of 1.0 since adapting the integration to work with those is out of scope: dedicated issue

Part of #4506

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
23940	1	23939	2291

View the top 1 failed test(s) by shortest run time

tests.integrations.wsgi.test_wsgi::test_auto_session_tracking_with_aggregates

Stack Traces | 0.095s run time

.../integrations/wsgi/test_wsgi.py:430: in test_auto_session_tracking_with_aggregates
    assert session_aggregates[0]["crashed"] == 1
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E   KeyError: 'crashed'

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