fix: remove allow_multiple_tool_calls config and simplify tool execution flow

This fixes a bug where the agent would stop responding abruptly without
calling final_output. The root cause was the allow_multiple_tool_calls
config option (default: false) which caused the agent to break out of
the streaming loop mid-stream after executing the first tool, losing
any subsequent content.

Changes:
- Remove allow_multiple_tool_calls config option entirely
- Always process all tool calls without breaking mid-stream
- Simplify system prompt generation (no longer needs boolean param)
- Let the stream complete fully before continuing to next iteration
- Change find_last_tool_call_start to find_first_tool_call_start
- Remove parser.reset() call on duplicate detection

Benefits:
- Simpler logic with less conditional branching
- No lost content after tool calls
- Consistent behavior for all users
- Reduced config complexity

crates/g3-ensembles/tests/integration_tests.rs

@@ -23,7 +23,6 @@ fallback_default_max_tokens = 8192
 enable_streaming = true
 timeout_seconds = 60
 auto_compact = true
-allow_multiple_tool_calls = false
 max_retry_attempts = 3
 autonomous_max_retry_attempts = 6