perf(client): static headers + bool fast-path in decodeResult

Two changes on the Call hot path:

* Replace httpReq.Header.Set("Content-Type", "application/json") (and Accept) with direct map writes against a package-level []string. Both keys are already canonical so the canonicalising path inside Header.Set was pure overhead; saves the per-call []string{val} allocation x2.

* Add a bool fast-path in decodeResult: ~60% of Telegram methods return bool, and the API emits the envelope with no whitespace, so a bytes.Equal check against the two canonical bodies short-circuits the generic Result[bool] Unmarshal entirely. any(true).(Resp) does not box thanks to Go's static bool interface values.

Combined effect on Call_BoolResponse: 18 -> 14 allocs/op, 634ns -> 526ns. DecodeResult_Bool isolation bench: 50ns / 2 allocs -> 2.87ns / 0 allocs.

client/multipart.go

@@ -69,7 +69,7 @@ func callMultipart[Resp any](ctx context.Context, b *Bot, method string, mp mult
 		return zero, &NetworkError{Err: err}
 	}
 	req.Header.Set("Content-Type", mw.FormDataContentType())
-	req.Header.Set("Accept", "application/json")
+	req.Header["Accept"] = headerJSONValue
 
 	resp, err := b.http.Do(req)
 	if err != nil {
@@ -125,7 +125,7 @@ func callMultipartRaw(ctx context.Context, b *Bot, method string, mp multipartRe
 		return nil, &NetworkError{Err: err}
 	}
 	req.Header.Set("Content-Type", mw.FormDataContentType())
-	req.Header.Set("Accept", "application/json")
+	req.Header["Accept"] = headerJSONValue
 
 	resp, err := b.http.Do(req)
 	if err != nil {