fix: reject unknown names in threat conditions

[rajanpanth]

Summary

• reject unknown names while validating threat condition expressions
• preserve names introduced by list/generator comprehensions
• add regression coverage for unknown names and comprehension variables

Testing

• py -3.12 -m compileall pytm\threat.py tests\test_pydantic_models.py
• git diff --check
• py -3.12 -m pytest tests\test_pydantic_models.py::TestConditionValidator -q in a temporary local venv

[NoodlesNZ]

Looks fine to me, although should include some additional changes:

_build_eval_globals should include an entry for LLM. While this doesn't break anything right now, if someone was writing threats using this element then it would error out.

We should also include a mention in CHANGELOG when we next release as this is a breaking change and may catch out legacy users.

[rajanpanth]

Both points addressed.

LLM is now in _build_eval_globals (2e40cf1). It was the only element class missing from the map, so conditions referencing it raised NameError even though eight shipped threats target the element.

The CHANGELOG entry is reworked in 97a4fbe. I'd originally filed the LLM addition itself under BREAKING, which was wrong — the breaking part is the strict rejection of unknown names. That's now its own entry describing what legacy users will actually hit (fail-fast when the Threat is constructed, instead of NameError later during evaluation) and how to resolve it, with the LLM fix moved under Fixed.

On the environment — fair. I have the suite running locally now. pytest tests/ gives 243 passed, 4 failed: test_dfd, test_dfd_colormap, test_dfd_duplicates_ignore and test_multilevel_dfd, all Windows path-separator diffs (pytm/images/... vs pytm\images\...). They fail identically on master, so they're not from this PR.

One thing I hit that may be worth its own issue: tests/test_pytmfunc.py can't be collected at all on Windows without PYTHONUTF8=1, because line 38 opens threats.json without an explicit encoding and cp1252 fails on it.