perf(client): pool req-body buffer + manual http.Request with cached URL

Two changes that together cut allocs/call from 15 to 13 (client-internal
bench) and per-call CPU from 600ns to 455ns (-24%) on the no-HTTP path:

1. Codec gets an optional BodyEncoder extension (MarshalTo io.Writer).
   When present, encodeJSONBody stream-encodes the request directly into
   a pooled *bytes.Buffer instead of allocating a [2-step] Marshal+Reader
   pair. DefaultCodec implements it via goccy/go-json.NewEncoder.
2. *Bot caches the parsed base URL on construction. buildRequest skips
   net/http.NewRequestWithContext for the common case and constructs
   *http.Request manually — clones the URL by value, sets the method
   path, and populates ContentLength + GetBody from the body's concrete
   type so RetryDoer's body-replay across attempts still works.

Cross-library bench (sendMessage round-trip vs httptest.Server): -2
allocs/call (104 -> 102), bytes -1.2%, time within noise (real HTTP
plumbing dominates). The CPU win is visible on synthetic stub-doer
benches and translates to lower GC pressure on sustained-throughput
workloads.

Slow-path fallback preserved for codecs that don't implement BodyEncoder
and for *Bot instances where url.Parse on the configured base failed —
they take the original NewRequestWithContext path.

client/call_bench_test.go

@@ -63,11 +63,14 @@ func BenchmarkEncodeJSONBody(b *testing.B) {
 	req := &benchSendReq{ChatID: 42, Text: "hello, world"}
 	b.ReportAllocs()
 	for b.Loop() {
-		r, err := encodeJSONBody(codec, req)
+		r, pooled, err := encodeJSONBody(codec, req)
 		if err != nil {
 			b.Fatal(err)
 		}
 		_ = r
+		if pooled != nil {
+			putReqBuf(pooled)
+		}
 	}
 }