chore: clean up dead code, fix broken test

- Remove unused methods: append_response_text, clear_response,
  has_pending_function_calls, take_function_calls
- Add #[allow(dead_code)] for intentionally kept future-use methods
  and response modification helpers
- Remove unused now_unix import from gemini.rs
- Fix test_modify_strips_all_tools: tools key is removed entirely
  when no custom tools provided, not left as empty array
- Zero warnings, 32 tests passing

src/api/gemini.rs

@@ -14,7 +14,7 @@ use tracing::info;
 
 use super::models::{lookup_model, DEFAULT_MODEL, MODELS};
 use super::polling::poll_for_response;
-use super::util::{err_response, now_unix};
+use super::util::err_response;
 use super::AppState;
 use crate::mitm::store::PendingToolResult;