fix: resolve all clippy warnings, formatting, and Mistral endpoint

- Fix Mistral provider base URL (missing /v1 prefix caused 404s)
- Resolve 55 clippy warnings across 28 warning types
- Apply cargo fmt to 44 formatting violations
- Remove unused imports (process_message, MultiObserver, VerboseObserver,
  ChatResponse, ToolCall, Path, TempDir)
- Replace format!+push_str with write! macro
- Fix unchecked Duration subtraction, redundant closures, clamp patterns
- Declare missing feature flags (sandbox-landlock, sandbox-bubblewrap,
  browser-native) in Cargo.toml
- Derive Default where manual impls were redundant
- Add separators to long numeric literals (115200 → 115_200)
- Restructure unreachable code in arduino_flash platform branches

All 1,500 tests pass. Zero clippy warnings. Clean formatting.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/gateway/mod.rs

@@ -810,7 +810,9 @@ mod tests {
                 .requests
                 .lock()
                 .unwrap_or_else(std::sync::PoisonError::into_inner);
-            guard.1 = Instant::now() - Duration::from_secs(RATE_LIMITER_SWEEP_INTERVAL_SECS + 1);
+            guard.1 = Instant::now()
+                .checked_sub(Duration::from_secs(RATE_LIMITER_SWEEP_INTERVAL_SECS + 1))
+                .unwrap();
             // Clear timestamps for ip-2 and ip-3 to simulate stale entries
             guard.0.get_mut("ip-2").unwrap().clear();
             guard.0.get_mut("ip-3").unwrap().clear();