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.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "cortiq-gateway"
-version = "0.2.45"
+version = "0.2.46"
 edition = "2021"
 rust-version = "1.75"
 license = "Apache-2.0"