perf(client): pool *bytes.Buffer for response body reads

Replace io.ReadAll(resp.Body) on the typed Call/callMultipart paths with a sync.Pool-backed bytes.Buffer + ReadFrom. Saves the 512B initial allocation that ReadAll grows from on every successful call.

The pool only covers paths whose decoder copies strings out of the input (decodeResult delegates to goccy/go-json, which copies). CallRaw and callMultipartRaw return slices that alias the buffer storage, so they keep the io.ReadAll path; pooling there would need a defensive copy that defeats the saving.

putRespBuf caps Cap() at 64 KiB before returning to the pool so a single oversized response (e.g. large getFile metadata) doesn't bloat the pool for the rest of the process.

Bench delta on Call_BoolResponse: 14 allocs -> 13 allocs, 1842B -> 1331B, 526ns -> 479ns. Same shape on Call_StructResponse (16 -> 15, 1973B -> 1462B).

client/multipart.go

@@ -1,6 +1,7 @@
 package client
 
 import (
+	"bytes"
 	"context"
 	"github.com/goccy/go-json"
 	"io"
@@ -81,12 +82,14 @@ func callMultipart[Resp any](ctx context.Context, b *Bot, method string, mp mult
 	}
 	defer func() { _ = resp.Body.Close() }()
 
-	raw, err := io.ReadAll(resp.Body)
-	if err != nil {
+	buf := respBufPool.Get().(*bytes.Buffer)
+	buf.Reset()
+	defer putRespBuf(buf)
+	if _, err := buf.ReadFrom(resp.Body); err != nil {
 		_ = pr.CloseWithError(err)
 		return zero, &NetworkError{Err: err}
 	}
-	return decodeResult[Resp](b.codec, raw)
+	return decodeResult[Resp](b.codec, buf.Bytes())
 }
 
 // callMultipartRaw is callMultipart's sibling that returns the raw result