feat: function-calling (MCP-style) tests plus ultra-hard tasks

Adds real OpenAI function-calling support end to end and two test tranches.

Client (internal/llm): Request.Tools advertises tools with tool_choice
auto; Response.ToolCalls carries the model's calls, decoding each
function's JSON-encoded argument string into an object (malformed
arguments preserved raw with Decoded=false). Verified live: the gateway
round-trips tools and the qwen/deepseek tool-call parsers emit the
expected format.

Runner: when a test declares Tools, it serializes the model's tool calls
into a canonical JSON envelope and scores that, not the free text.

Eval (toolcall.go): ToolCalled, NoToolCalled, ToolCallWithArgs (subset
match, number/string coercion), ToolSequence, FirstToolIs, ToolNotCalled,
ToolArgValuesCover - all parse the envelope, no llm dependency.

Tests: 12 function-calling tests (tools/function-calling) covering tool
selection, argument extraction, enum constraints, chained and parallel
calls, no-tool-needed and missing-param abstention, destructive-tool
safety, and name-vs-description traps; 8 ultra-hard single-shot tasks in
existing subcategories. Ground-truth self-consistency tests assert every
documented answer and ideal tool envelope scores 1.0.

internal/eval/toolcall_test.go

@@ -0,0 +1,124 @@
+package eval
+
+import (
+	"context"
+	"testing"
+)
+
+func TestToolCalled(t *testing.T) {
+	tests := []struct {
+		name     string
+		response string
+		want     float64
+	}{
+		{"called", `{"tool_calls":[{"name":"get_weather","arguments":{"city":"Paris"}}]}`, 1},
+		{"case-insensitive", `{"tool_calls":[{"name":"Get_Weather","arguments":{}}]}`, 1},
+		{"among several", `{"tool_calls":[{"name":"a","arguments":{}},{"name":"get_weather","arguments":{}}]}`, 1},
+		{"not called", `{"tool_calls":[{"name":"other","arguments":{}}]}`, 0},
+		{"none called", `{"tool_calls":[]}`, 0},
+		{"malformed envelope", `not json`, 0},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := ToolCalled("get_weather").Evaluate(context.Background(), tt.response).Value
+			if got != tt.want {
+				t.Errorf("= %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestNoToolCalled(t *testing.T) {
+	tests := []struct {
+		name     string
+		response string
+		want     float64
+	}{
+		{"none called is correct", `{"tool_calls":[],"content":"the answer is 4"}`, 1},
+		{"missing tool_calls key is empty", `{"content":"4"}`, 1},
+		{"called a tool is wrong", `{"tool_calls":[{"name":"calc","arguments":{}}]}`, 0},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := NoToolCalled().Evaluate(context.Background(), tt.response).Value
+			if got != tt.want {
+				t.Errorf("= %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestToolCallWithArgs(t *testing.T) {
+	tests := []struct {
+		wantArgs map[string]any
+		name     string
+		response string
+		wantName string
+		want     float64
+	}{
+		{
+			name:     "exact match",
+			response: `{"tool_calls":[{"name":"book","arguments":{"city":"NYC","nights":3}}]}`,
+			wantName: "book", wantArgs: map[string]any{"city": "NYC", "nights": 3}, want: 1,
+		},
+		{
+			name:     "subset match ignores extra args",
+			response: `{"tool_calls":[{"name":"book","arguments":{"city":"NYC","nights":3,"pets":true}}]}`,
+			wantName: "book", wantArgs: map[string]any{"city": "NYC"}, want: 1,
+		},
+		{
+			name:     "number-as-string coerced",
+			response: `{"tool_calls":[{"name":"book","arguments":{"nights":"3"}}]}`,
+			wantName: "book", wantArgs: map[string]any{"nights": 3}, want: 1,
+		},
+		{
+			name:     "wrong value",
+			response: `{"tool_calls":[{"name":"book","arguments":{"city":"LA"}}]}`,
+			wantName: "book", wantArgs: map[string]any{"city": "NYC"}, want: 0,
+		},
+		{
+			name:     "missing arg",
+			response: `{"tool_calls":[{"name":"book","arguments":{"nights":3}}]}`,
+			wantName: "book", wantArgs: map[string]any{"city": "NYC"}, want: 0,
+		},
+		{
+			name:     "right name malformed args",
+			response: `{"tool_calls":[{"name":"book","arguments":null,"arguments_malformed":true}]}`,
+			wantName: "book", wantArgs: map[string]any{"city": "NYC"}, want: 0,
+		},
+		{
+			name:     "wrong tool",
+			response: `{"tool_calls":[{"name":"other","arguments":{"city":"NYC"}}]}`,
+			wantName: "book", wantArgs: map[string]any{"city": "NYC"}, want: 0,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := ToolCallWithArgs(tt.wantName, tt.wantArgs).Evaluate(context.Background(), tt.response).Value
+			if got != tt.want {
+				t.Errorf("= %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestToolSequence(t *testing.T) {
+	tests := []struct {
+		name     string
+		response string
+		want     []string
+		wantVal  float64
+	}{
+		{"exact order", `{"tool_calls":[{"name":"a","arguments":{}},{"name":"b","arguments":{}}]}`, []string{"a", "b"}, 1},
+		{"wrong order", `{"tool_calls":[{"name":"b","arguments":{}},{"name":"a","arguments":{}}]}`, []string{"a", "b"}, 0},
+		{"wrong count", `{"tool_calls":[{"name":"a","arguments":{}}]}`, []string{"a", "b"}, 0},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := ToolSequence(tt.want...).Evaluate(context.Background(), tt.response).Value
+			if got != tt.wantVal {
+				t.Errorf("= %v, want %v", got, tt.wantVal)
+			}
+		})
+	}
+}