Adds 20 scenarios that exercise ConversationSettings clamping,
ConversationBlock line counting, ConversationStream bootstrap / clear /
extend / render / pruning behaviour, and load_conversation_settings
fallback paths directly. The pre-existing single Behave scenario
covered ConversationStream only through the TUI app's
_append_conversation_block, leaving most of the new conversation
module unmeasured and pulling overall coverage below the 97%
threshold (the lone failing CI gate).
ISSUES CLOSED: #6350
drew
HAL9000