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/codec.go

@@ -1,7 +1,11 @@
 // Package client provides HTTP client primitives for the Telegram Bot API.
 package client
 
-import "github.com/goccy/go-json"
+import (
+	"io"
+
+	"github.com/goccy/go-json"
+)
 
 // Codec encodes/decodes JSON payloads exchanged with the Telegram Bot API.
 // The default implementation wraps goccy/go-json. Users may plug in
@@ -12,6 +16,15 @@ type Codec interface {
 	Unmarshal(data []byte, v any) error
 }
 
+// BodyEncoder is an optional Codec extension that encodes directly into
+// an io.Writer, skipping the intermediate []byte that Marshal returns.
+// Call uses this when present to avoid allocating the marshal result and
+// the bytes.Reader that wraps it; codecs without it fall through to
+// Marshal + bytes.NewReader.
+type BodyEncoder interface {
+	MarshalTo(w io.Writer, v any) error
+}
+
 // DefaultCodec wraps goccy/go-json. It is the zero-value safe default.
 type DefaultCodec struct{}
 
@@ -20,3 +33,10 @@ func (DefaultCodec) Marshal(v any) ([]byte, error) { return json.Marshal(v) }
 
 // Unmarshal calls json.Unmarshal.
 func (DefaultCodec) Unmarshal(data []byte, v any) error { return json.Unmarshal(data, v) }
+
+// MarshalTo encodes v into w via goccy/go-json's streaming encoder. The
+// trailing newline that Encoder appends is valid JSON whitespace and is
+// accepted by Telegram's parser.
+func (DefaultCodec) MarshalTo(w io.Writer, v any) error {
+	return json.NewEncoder(w).Encode(v)
+}