MAINT: Modernize Optional/Union typing to PEP 604 syntax

[romanlutz]

Description

Phase 1 of a typing-syntax cleanup. The codebase mixes legacy typing.Optional / typing.Union with the modern PEP 604 X | None / A | B syntax that our style guide requires (and that ruff's FA rule already enforces). This PR modernizes a first batch of 10 priority files so they are consistent with that convention. No behavior changes.

What changed:

• Replaced Optional[T] with T | None and Union[A, B] with A | B in annotations and in docstring type references.
• Added from __future__ import annotations to the 8 files whose annotations needed it, so the unions can be written unquoted (X | None) rather than as quoted forward-ref strings. This avoids the runtime TypeError you get from evaluating "X" | None when a referenced name is only imported under TYPE_CHECKING.
• Moved annotation-only imports into TYPE_CHECKING blocks to satisfy the repo's flake8-type-checking (TCH) rules, keeping runtime-needed imports at module level. This matches the pattern already used by the ~35 existing future-annotations modules.
• Two files (memory/storage/storage.py, memory/storage/serializers.py) only needed docstring type-reference updates, so they get no import or annotation changes.

Worth noting: two candidate files were intentionally left untouched because they use Union at runtime (backend/services/converter_service.py does origin is Union checks; scenario/core/attack_technique_factory.py does the same), so converting them would change behavior. Verified that the introspection consumers of these annotations (brick_contract.enforce_keyword_only_init, the converter-service catalog, and the factory's get_type_hints call) are unaffected by the switch to deferred annotations.

Tests and Documentation

No new tests are needed since this is a no-behavior-change typing refactor. Validated with the existing tooling:

• ruff check and ruff format --check pass on all touched files.
• ty check passes (confirms every unquoted forward-ref name resolves via its TYPE_CHECKING import).
• Import-tested all affected modules plus the two intentionally-unchanged runtime-Union modules.
• Ran the relevant unit suites (scorers, prompt converter, atomic attack, conversation manager, multi-prompt sending, tree of attacks, attack technique factory, converter service) after merging latest main: all passing.

No documentation (JupyText) changes are required for this change.