fix: preserve agent inventories when serializing GameState

[tsubasakong]

Problem

Inventory (fle/env/entities.py) is a pydantic model with extra="allow", so item counts are stored in __pydantic_extra__ and inventory.__dict__ is always {}. Two call sites serialize inventories via __dict__:

• GameState.to_raw() (fle/commons/models/game_state.py) — every saved game state gets "inventories": [{}]
• the reset tool client (fle/env/tools/admin/reset/client.py) — FactorioInstance.reset(game_state) passes inventories through this path, so every GameState restore silently resets all agents to empty inventories

This appears to be a regression: Inventory previously populated self.__dict__ directly in a custom __init__ (still visible, commented out, in fle/env/entities.py), which is why __dict__-based serialization used to work — and why it broke silently when that constructor was removed.

Fix

Serialize via model_dump() (which includes pydantic extras), with items() / __dict__ fallbacks for non-pydantic inputs. Plain-dict inventories are unaffected.

Tests

Adds server-free regression tests under tests/unit/ (with a local conftest that shadows the repo-level autouse fixture requiring a live Factorio server):

• to_raw preserves pydantic-extra inventories
• plain-dict inventories still pass through
• multi-agent to_raw → parse_raw round-trip
• the reset client sends actual item counts in its JSON payload

Negative control: on the unfixed code these tests fail 3/4 (the only pass is the plain-dict case, which was never broken); with the fix, 4/4 pass.

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