fix: preserve current input when maxContextMessageCount is 0 (#706)

The limit now applies to history messages only, not the current user input.
- maxContextMessageCount = 0: only sends current input (no history)
- maxContextMessageCount = 2: sends 2 history messages + current input

Fixes #3620

(cherry picked from commit 55a809398636c89163e874dd71b64f5630f96ef8)