Prevent players from getting into the game without a team when both teams are full

[belomaxorka]

Fixes #929
Fixes #932

What

Players could get into the game with m_iTeam == UNASSIGNED when both teams are full — a teamless "player" that can move, shoot and pick up weapons, shows no team on the radar/scoreboard and is hostile to everyone. Reproducible with mp_auto_join_team 1 + mp_forcerespawn 1 on a server where bots fill both teams (bot_auto_vacate 0), and the same fall-through is reachable in the vanilla menu flow via "Auto-select" (slot 5).

Why (Root cause)

Two independent paths lead to spawning a player without a team:

1. HandleMenu_ChooseTeam() with MENU_SLOT_TEAM_RANDOM: when SelectDefaultTeam() returns UNASSIGNED (both teams full) and no bot can be vacated, the function falls through — TeamFull(UNASSIGNED) and TeamStacked(UNASSIGNED, ...) both return FALSE — so it assigns m_iTeam = UNASSIGNED and returns TRUE. The comment "If the code gets this far, the team is not TEAM_UNASSIGNED" does not hold on this path. The player then goes through the normal joining flow (HandleMenu_ChooseAppearance() → GetIntoGame()), where FPlayerCanRespawn() always passes with mp_forcerespawn > 0, resulting in an immediate Spawn() with no team.

2. PlayerRespawnThink(): the guard GetObserverMode() != OBS_NONE && (m_iTeam == UNASSIGNED || m_iTeam == SPECTATOR) does not protect a player sitting at the team select menu (Menu_ChooseTeam / PICKINGTEAM, deadflag == DEAD_DEAD since ClientPutInServer(), observer mode OBS_NONE): the menu check on the next line only covers Menu_ChooseAppearance / SHOWTEAMSELECT, so mp_forcerespawn schedules and executes a respawn for a teamless player anyway.

How it works

• HandleMenu_ChooseTeam(): deny auto-select (return FALSE) when the team is still UNASSIGNED after the vacate attempt — the player stays at the team select menu, consistent with explicitly picking a full team (slots 1/2 are already denied by the TeamFull() check) and with the bot path, which already refuses to add a bot in this case ("Could not add bot to the game: Team is full"). Guarded by REGAMEDLL_FIXES since the fall-through exists in the vanilla code.
• PlayerRespawnThink(): never force-respawn players without a team (UNASSIGNED/SPECTATOR) regardless of observer mode. Unlike PlayerDeathThink() (which is gated by m_iJoiningState != JOINED), this function is reached from PreThink() for players in any joining state, hence the stronger guard.

Edge cases

• All human join paths (VGUI menu, jointeam console command, mp_auto_join_team) go through HandleMenu_ChooseTeam(), so the fix covers them all.
• Round restart (RoundRespawn()) already skips teamless players, and the bot add path already denies UNASSIGNED — behavior is now consistent everywhere.
• Vanilla builds are unaffected: PlayerRespawnThink() is REGAMEDLL_ADD-only and the HandleMenu_ChooseTeam() change is under REGAMEDLL_FIXES.

Tested

On a 24-slot Windows server (cs_office, 10+10 spawn points), bot_quota 32; bot_stop 1; bot_join_after_player 0; bot_auto_vacate 0; mp_forcerespawn 1; mp_auto_join_team 1; humans_join_team any: bots fill both teams; without the fix a connecting player spawns into the world with no team (immediately via auto-join, or after mp_forcerespawn seconds when left at the team menu); with the fix the player stays at the team select menu. Manually choosing T/CT/Spectator, team changes and bot vacating (bot_auto_vacate 1) behave as before.