refactor(agent): replace XML api_call with native function calling

Migrate the Telegram bot agent from an XML tag hack (<api_call>) to
OpenAI-native function calling via CallWithRequestFull.

Key changes:
- mcp/interface.go: add parseMCPResponseFull to clientHooks interface
- mcp/client.go: route callWithRequestFull through hooks for overridability
- mcp/claude_client.go: override parseMCPResponseFull for Claude response
  format (tool_use blocks instead of choices[].message.tool_calls)
- telegram/agent/agent.go: rewrite Run() to use CallWithRequestFull;
  define api_request tool with JSON Schema; implement tool-call loop
  with role="tool" result messages; remove XML parsing entirely
- telegram/agent/apicall.go: remove parseAPICall (dead code)
- telegram/agent/prompt.go: simplify — remove XML format instructions,
  replace with concise api_request tool usage instructions
- telegram/agent/agent_test.go: rebuild all tests using LLMResponse
  objects; add TestNarrationStructurallyImpossible, TestOnChunkCalledWithFinalReply,
  TestToolCallIDPropagated; remove XML-specific tests

Architecture advantage: with native function calling, the LLM returns
EITHER ToolCalls OR Content — never both. Narration is now structurally
impossible at the protocol level, not just enforced by prompt rules.

All 11 agent tests pass. mcp package tests pass.

mcp/interface.go

@@ -15,6 +15,11 @@ type AIClient interface {
 	// onChunk is called with the full accumulated text so far (not raw deltas).
 	// Returns the complete final text when done.
 	CallWithRequestStream(req *Request, onChunk func(string)) (string, error)
+	// CallWithRequestFull returns both text content and tool calls.
+	// Use this when the request includes Tools — the LLM may respond with
+	// either a plain text reply (LLMResponse.Content) or tool invocations
+	// (LLMResponse.ToolCalls), but not both.
+	CallWithRequestFull(req *Request) (*LLMResponse, error)
 }
 
 // clientHooks internal hook interface (for subclass to override specific steps)
@@ -30,5 +35,6 @@ type clientHooks interface {
 	setAuthHeader(reqHeaders http.Header)
 	marshalRequestBody(requestBody map[string]any) ([]byte, error)
 	parseMCPResponse(body []byte) (string, error)
+	parseMCPResponseFull(body []byte) (*LLMResponse, error)
 	isRetryableError(err error) bool
 }