Complete type hinting across scienceworld package; add opt-in mypy tox env

[pangwangshu]

Summary

Fixes #58.

Type hinting was already partially added to ScienceWorldEnv's snake_case API in earlier commits. This PR completes the coverage and, per MarcCote's comment on the issue, wires up an opt-in mypy check without making it mandatory.

• Add type hints to scienceworld/utils.py (infer_task, snake_case_deprecation_warning), which had none.
• Add type hints to the remaining untyped surface of scienceworld/scienceworld.py: the deprecated camelCase wrapper methods, __del__, getObjectTree, get_max_variations's parameter, and the whole BufferedHistorySaver class.
• Fix two bugs surfaced by mypy while typing:
  • getRunHistorySize() called a nonexistent get_run_historySize() method (should be get_run_history_size()) — would have raised AttributeError for any camelCase-API caller.
  • ScienceWorldEnv.__init__'s taskName/serverPath params were implicitly Optional[str].
• Add a [testenv:mypy] section to tox.ini, deliberately not part of the default envlist/CI matrix, so it's runnable via tox -e mypy without gating builds — matching the "not mandatory for now" ask in the issue thread.

Test plan

• mypy scienceworld — no issues found
• flake8 scienceworld --max-line-length 120 — clean
• Relevant pre-commit checks (trailing-whitespace, end-of-file-fixer, codespell) — clean on changed files
• All changed files parse/compile (py_compile)
• Full pytest suite — not run locally (no JVM available in the sandbox this was developed in); relies on CI, which builds the Scala JAR

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