fix(routing): offered tools must not make a local model unroutable

Verified against a real Open WebUI container: it attaches its ~26 built-in
tools (knowledge bases, notes, calendar, ...) to every single message, and
resolve() treated that offer as a hard capability requirement. On a pool of
tools-less local models a plain "hello" therefore failed with "No healthy
model available that satisfies required capabilities", and a pinned local
model was reported as "not available - not in the pool (deleted?)".

In the OpenAI contract `tools` is an offer; only tool_choice: "required" (or
a named function) makes the call impossible without them. Keep preferring a
tool-capable model, but fall back to serving the request with the tools
stripped - together with tool_choice/parallel_tool_calls, which a provider
rejects on their own. A pin whose only shortcoming is tool support is now
honoured; an unknown pin and tool_choice: "required" still fail loudly.

The downgrade is never silent: X-Cortiq-Tools-Dropped: true, plus
cortiq.tools_dropped in the echo block.

src/model.rs

@@ -113,6 +113,11 @@ pub struct RouteInfo {
     pub cost_usd: f64,
     #[serde(default)]
     pub failover: bool,
+    /// The client offered `tools` but the model that answered cannot use them,
+    /// so they were dropped from the upstream call (see `Pipeline::resolve`).
+    /// Surfaced as `X-Cortiq-Tools-Dropped` so a silent downgrade is debuggable.
+    #[serde(default)]
+    pub tools_dropped: bool,
 }
 
 #[derive(Clone, Debug, Serialize)]