add CDirector::AddSurvivorBot native for spawn survivors (L4D2 only), called via sb_add command

[spumer]

Summary

Adds L4D2Direct_AddSurvivorBot(int characterType) native — a direct wrapper around
CDirector::AddSurvivorBot(SurvivorCharacterType) that allows plugins to spawn a new survivor bot with a specific character type.

Motivation

There was no clean way to programmatically add a survivor bot in Left4DHooks. Existing alternatives
have significant drawbacks:

• ServerCommand("sb_add") — executes asynchronously (next server tick), no return value, no immediate
  control over the created bot
• CreateFakeClient() + manual setup — low-level and fragile, requires manually handling team
  assignment, character selection, spawning, and many edge cases the engine normally handles

L4D2Direct_AddSurvivorBot calls the engine's own CDirector::AddSurvivorBot — the same function used by
the sb_add console command — providing synchronous execution with proper character selection through
the game's native code path.

API

/**
 * Adds a survivor bot to the game
 *
 * @param characterType			See SurvivorCharacter_* enum values:
 *								0-3 = Map's survivor set characters (set-dependent, see enum)
 *								4-7 = L4D1 characters (Bill, Zoey, Francis, Louis — always)
 *								8   = Random (0-3, guaranteed to create)
 *
 * @remarks Indices 0-3 resolve to different characters depending on the map's survivor set
 *          (L4D2 maps: Nick/Rochelle/Coach/Ellis, L4D1 maps: Bill/Zoey/Louis/Francis).
 * @remarks Characters 4-7: engine checks for duplicates, silently does nothing if taken.
 * @remarks Characters 0-3: engine does NOT check for duplicates, will create a second bot with same character.
 *
 * @return					0
 */
native int L4D2Direct_AddSurvivorBot(int characterType = 0);