steam: harden VDF parser & library detection; add tests

[astrovm]

Motivation

• Make Steam VDF parsing and library detection more robust against malformed or weird real-world VDF files and remove a small dead/no-op branch to keep code clearer.
• Add unit coverage for edge cases observed in the wild where libraryfolders entries or their apps child are not maps.

Description

• Simplified vdf::Parser::parse_map by removing the found_closing_brace flag and returning early on EOF or closing-brace conditions, making the control flow clearer (src/steam/vdf.rs).
• Removed an empty/no-op if kind_found {} block and tightened the conditional branches in detect_games_from_parsed_vdfs to only log when nothing was found and otherwise dedupe inaccessible entries (src/steam/library.rs).
• Added two unit tests to cover malformed VDF structures: test_find_game_skips_non_map_library_entries and test_find_game_skips_non_map_apps to ensure such entries are safely ignored (src/steam/library.rs).

Testing

• Ran cargo fmt --all which completed successfully.
• Attempted cargo test -q but the test run failed in this environment because glib-2.0 development pkg-config metadata is missing, so glib-sys cannot build and the full test suite could not be executed.
• Also attempted PKG_CONFIG_PATH=/usr/lib/x86_64-linux-gnu/pkgconfig:/usr/share/pkgconfig cargo test -q which failed with the same missing glib-2.0.pc blocker; the new tests are present in the tree and should pass when system dev deps are available.

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Codex Task