steam/vdf: reject truncated maps and trailing data; add edge-case tests

[astrovm]

Motivation

• The VDF parser accepted truncated maps or allowed trailing tokens after a valid root, which can hide malformed Steam VDF files and produce ambiguous detection results.

Description

• Make parse_map in src/steam/vdf.rs fail (return None) when a closing } is not found instead of silently accepting EOF.
• Tighten top-level parsing in parse to call skip_whitespace() and reject the input if any non-whitespace remains after a parsed root object.
• Update tests in src/steam/vdf.rs to assert strict behavior: change the unclosed-brace test to expect failure and add test_parse_rejects_trailing_non_whitespace_text and test_parse_rejects_second_root_object to cover trailing garbage and multiple top-level objects.
• Minor formatting adjustments to keep code tidy.

Testing

• Ran cargo fmt --all successfully.
• Attempted PKG_CONFIG_PATH=/usr/lib/x86_64-linux-gnu/pkgconfig:/usr/share/pkgconfig cargo test -q steam::vdf::tests, but the test run cannot complete in this environment due to missing system GNOME/GLib pkg-config metadata (glib-2.0.pc), so unit tests should be validated in a developer environment or CI where the native glib-2.0 dev package is installed.

---

Codex Task