fix(agent): count compression restarts toward retry limit

[teknium1]

Summary

When context overflow triggers compression, the outer retry loop restarts via continue without incrementing retry_count. If compression reduces messages but not enough to fit the context window, this creates an infinite loop burning API credits:

API call → overflow → compress → retry → overflow → compress → ... (forever at retry_count=0)

compression_attempts limits inner compression attempts, but the outer while retry_count < max_retries loop never exits.

Fix

Increment retry_count on compression restarts so the loop has a hard exit after max_retries total attempts. Also explicitly resets restart_with_compressed_messages at the continue site for clarity.

Note: restart_with_length_continuation (truncated response continuation) correctly does NOT increment retry_count — that's normal behavior, not an error.

Cherry-picked from PR #2766 by @dieutx.