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/llm/llm.go

@@ -13,10 +13,33 @@ type Message struct {
 	Content string
 }
 
-// Request is one chat-completion request sent to a model.
+// Tool is one function the model may call, in OpenAI function-calling
+// shape. Parameters is a JSON Schema object describing the arguments.
+type Tool struct {
+	Parameters  map[string]any
+	Name        string
+	Description string
+}
+
+// ToolCall is one function invocation the model emitted. Arguments is the
+// decoded argument object (OpenAI sends it as a JSON-encoded string on the
+// wire; the client decodes it before returning). Decoded is false when the
+// model produced malformed argument JSON, in which case RawArguments holds
+// the undecodable string for scoring.
+type ToolCall struct {
+	Arguments    map[string]any
+	Name         string
+	RawArguments string
+	Decoded      bool
+}
+
+// Request is one chat-completion request sent to a model. When Tools is
+// non-empty the client advertises them with tool_choice "auto", so the
+// model decides whether and which to call.
 type Request struct {
 	Model       string
 	Messages    []Message
+	Tools       []Tool
 	MaxTokens   int
 	Temperature float64
 }
@@ -25,6 +48,7 @@ type Request struct {
 type Response struct {
 	Text             string
 	FinishReason     string
+	ToolCalls        []ToolCall
 	PromptTokens     int
 	CompletionTokens int
 	Latency          time.Duration