refactor(chat): clean up various /compact issues

[brandonskiser]

Description of changes:

• Addressing comments raised in feat: Introduce /compact command #1128
• More accurately estimate the character count calculation for the conversation history
• Update some UI points
• Other misc code refactoring

Couple issues remain:

1. There are two consecutive AssistantResponseMessages in the history after running /compact. This is because we push the assistant message on ResponseEvent::EndStream and again later in the same function. We don't get any validation errors for this, so this is apparently acceptable for now.
2. The calculate_char_count function does not take context files into account. This is because we send the result of as_sendable_conversation_state to the model which includes the result of context_messages, but calculate_char_count does not use this output. Instead, as_sendable_conversation_state should be heavily refactored to not mutate ConversationState, so we can decouple our actual request data from the internal app representation (ie, ConversationState) more appropriately.

Since we only underestimate rather than overestimate token count, this should still be fine at the moment.

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 79.45205% with 15 lines in your changes missing coverage. Please review.

Project coverage is 13.94%. Comparing base (d13b4c1) to head (6edcfd8).

Files with missing lines	Patch %	Lines
crates/q_cli/src/cli/chat/mod.rs	29.41%	12 Missing ⚠️
crates/q_cli/src/cli/chat/conversation_state.rs	94.64%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1169      +/-   ##
==========================================
+ Coverage   13.92%   13.94%   +0.02%     
==========================================
  Files        2363     2363              
  Lines      204441   204483      +42     
  Branches   184805   184847      +42     
==========================================
+ Hits        28466    28523      +57     
+ Misses     174575   174560      -15     
  Partials     1400     1400              

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