chore(core): remove stale blockbuster allowlist for deleted context module (#36168)

Closes #29530

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Remove a stale BlockBuster allowlist entry in `conftest.py` referencing
`aconfig_with_context` — the function and its containing module
(`langchain_core/beta/runnables/context.py`) were deleted in `fded6c6b1`
(Sep 2025, #32850). Spotted by @antonio-mello-ai in #29530.