security: remove unpatched NLTK dependency chain

[kmadan]

Summary

Fix Dependabot alert #199 (NLTK: Model-artifact APIs bypass pathsec and touch files outside allowed roots).

GitHub's advisory affects released NLTK versions through 3.10.3. NLTK upstream has merged the remediation for the model-artifact save APIs, but as of this PR there is no patched release available to depend on.

AICertify does not import NLTK directly. The vulnerable package is present in the default dependency graph through LangFair (langfair -> nltk and langfair -> rouge-score -> nltk).

Changes

• remove langfair from AICertify's default dependencies
• remove the explicit nltk>=3.10.3 floor, which still selects an affected release
• remove the existing poetry.lock, because it records NLTK 3.10.3 and would continue to ship/scan the vulnerable graph; CI now resolves the safe dependency set from pyproject.toml
• add a CI assertion that fails if NLTK re-enters the default install before a patched upstream release is available

Why not bump NLTK?

There is currently no released version newer than 3.10.3 containing the upstream fix. Raising the lower bound to 3.10.3 does not remediate this advisory.

Upstream remediation commit: nltk/nltk@2a92b71827d754ae8920261e7ed0c4bb283ab2d7.

Functional impact

The OPA/compliance core, reports, DeepEval integration, CLI, and other AICertify functionality remain in the default install. LangFair-backed fairness evaluation is temporarily no longer installed automatically. Its existing runtime fallback already treats LangFair as optional when unavailable.

Once NLTK publishes a fixed release, LangFair can be restored to the default dependency set with a safe NLTK floor and the temporary CI exclusion can be removed.

Verification

CI now verifies that a fresh default Poetry environment contains no NLTK package, then runs the existing pre-commit checks, package import check, OPA setup, and full test suite.

Closes/addresses Dependabot alert #199.