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/mitm/modify.rs

@@ -635,9 +635,9 @@ mod tests {
         let modified = modify_request(&bytes, None).unwrap();
         let result: Value = serde_json::from_slice(&modified).unwrap();
 
-        let tools = result["request"]["tools"].as_array().unwrap();
-        // With no ToolContext, tools should just be stripped (empty)
-        assert!(tools.is_empty(), "all tools should be stripped");
+        // With no ToolContext, tools should be removed entirely
+        assert!(result["request"]["tools"].is_null() || result.pointer("/request/tools").is_none(),
+            "tools should be removed when no custom tools provided");
     }
 
     #[test]
@@ -734,6 +734,7 @@ mod tests {
 // ─── Response modification ──────────────────────────────────────────────────
 
 /// Rewrite an SSE response chunk to replace `functionCall` parts with text,
+#[allow(dead_code)]
 /// so the LS doesn't see tool calls for tools it doesn't manage.
 ///
 /// The MITM intercept layer has already captured the function call data
@@ -810,6 +811,7 @@ pub fn modify_response_chunk(chunk: &[u8]) -> Option<Vec<u8>> {
     }
 }
 
+#[allow(dead_code)]
 /// Find the end of a JSON object starting at the given string.
 /// Returns the index past the closing brace.
 fn find_json_end(s: &str) -> Option<usize> {
@@ -845,6 +847,7 @@ fn find_json_end(s: &str) -> Option<usize> {
     None
 }
 
+#[allow(dead_code)]
 /// Rebuild chunked encoding from a modified response body.
 /// Takes the full text (which contains old chunk sizes) and rebuilds
 /// with correct sizes.
@@ -881,6 +884,7 @@ fn rechunk_response(text: &str) -> String {
 ///
 /// Handles both Gemini public API format (`{"candidates":[...]}`) and
 /// internal LS format (`{"response":{"candidates":[...]}}`).
+#[allow(dead_code)]
 fn rewrite_function_calls_in_response(json: &mut Value) -> bool {
     let mut changed = false;