`src/middleware/unknown_tool.rs` — `UnknownToolMiddleware`

Turns bare “tool not found” errors into teachable moments: "Tool not found: redd_file. Available: ... Did you mean 'read_file'?"

Key items

• UnknownToolMiddleware::new(registry) (threshold 0.4) / with_threshold(0.0..=1.0).
• similarity(a, b) -> f64 — LCS-based + prefix bonus, public static.

Behavior notes

• Detection is deliberately specific: single text part, is_error, containing both “not found” and “tool” — so “file not found” never triggers it.
• The suggestion lookup uses the post-dispatch ctx.tool_name — correct even behind renaming middleware (pinned by test).
• Ties break lexicographically — deterministic and order-independent.
• Names the model was never shown are pre-answered by the engine (never dispatched) and never reach this layer.

Deep dive: Middleware.