Remove SteamKit2 in favor of steamcmd

[JantsoP]

Remove SteamKit2 in favor of official SteamCMD

Description

Removes the BytexDigital.Steam git submodule and all SteamKit2-based
download/update logic, replacing it with a first-party SteamCmd service
layer (FASTER/Services/SteamCmd/) that drives Valve's real steamcmd.exe
through the native Windows pseudoconsole (ConPTY) and parses its console
output into structured events.

Key changes:

• New SteamCmdInstaller bootstraps/updates SteamCMD automatically if it's
  missing from the configured path — no manual setup required.
• New SteamCmdSession / SteamCmdPseudoConsole host a long-lived SteamCMD
  process and stream its stdout/stdin through ConPTY, since SteamCMD requires
  a real console to prompt for passwords/Steam Guard.
• New SteamCmdOutputParser incrementally turns SteamCMD's raw console text
  into typed events (login prompts, Steam Guard/mobile confirmation prompts,
  per-item download progress, success/failure, app update percentages, etc.),
  redacting secrets from anything it emits.
• New SteamCmdClient exposes DownloadWorkshopItemsAsync and
  UpdateServerAsync, and downloads multiple Workshop items in a single
  SteamCMD session instead of one process per mod.
• New WorkshopContentMirror copies each completed SteamCMD Workshop
  download into FASTER's mod staging directory and atomically swaps it in,
  so an interrupted copy never leaves a half-updated mod.
• SteamUpdaterViewModel rewritten around the new client: retries failed
  Workshop items, supports Steam Guard/mobile-approval prompts via dialog,
  reports live download/update percentages to the progress bar, and now
  writes a detailed, timestamped SteamCMD session log to
  %LocalAppData%\FASTER\Logs\SteamCmd.log for troubleshooting without
  flooding the on-screen console.
• Removed SteamWebApi's download/auth responsibilities — the Web API key
  is now optional and only used for Workshop metadata/update-timestamp
  lookups, not authentication.
• Settings, Setup, and Updater views/UX updated for the new SteamCMD path
  setting and login flow.
• README updated with the new prerequisites (Windows 10 1809+/Server 2019+
  for ConPTY) and a full explanation of the SteamCMD update workflow.

Motivation and Context

The previous SteamKit2 (BytexDigital.Steam) integration required workarounds
to authenticate and pull Workshop content, downloaded mods one at a time, and
was fragile around Steam Guard/2FA flows. Valve's own steamcmd.exe is the
officially supported way to automate Steam/Workshop operations, downloads
multiple Workshop items in one session, and is noticeably faster in practice.
This also drops an entire external git submodule dependency, simplifying the
build and reducing maintenance surface.

How Has This Been Tested?

• Added unit tests: SteamCmdCommandBuilderTests, SteamCmdOutputParserTests
  (parsing prompts, Steam Guard/mobile confirmation, workshop success/failure,
  progress lines, secret redaction), SteamCmdClientTests, and
  WorkshopContentMirrorTests.
• Added an opt-in SteamCmdLiveSmokeTests ([Category("SteamCmdLive")])
  that exercises a real SteamCMD handshake up to the authenticated
  server-update prompt; skipped unless FASTER_STEAMCMD_SMOKE_ROOT is set,
  so it doesn't run in normal CI.
• Manually verified end-to-end on Windows: built a Release x64 self-contained
  publish, ran a real Arma 3 dedicated server update (Public branch) through
  the app, confirmed cached-login authentication, live progress bar updates
  matching SteamCMD's reported percentages, and a clean
  "Arma 3 Dedicated Server update completed" result — cross-checked line by
  line against the new SteamCmd.log output.
• Manually verified a multi-item Workshop batch download completes and stages
  each mod correctly.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

This removes the BytexDigital.Steam submodule and changes how Workshop
downloads authenticate (a real Steam account is now required instead of
relying solely on a Web API key), so existing users need to reconfigure
their SteamCMD path/Steam login on first run after upgrading.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• [x ] I have read the CONTRIBUTING document.

[sonarqubecloud]

Quality Gate failed

Failed conditions
E Reliability Rating on New Code (required ≥ A)

See analysis details on SonarQube Cloud

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[jupster]

It might take a bit of time for this to be gone through due to how busy I know @Foxlider is and ive been pretty busy lately as well. Really appreciate the PR.

Ive change your push branch to update as it won't go straight to master.

Make sure you have a read of the contributing file.

[JantsoP]

I have tested this on my machine and the migration was flawless. I had 97 out of 126 mods which were not loaded. With current version, it would take me hours to download them all. Now it took few minutes (fast internet and all)