fix: Find the receipts URL from the connection URL

We were defaulting to the production URL unless a `receiptsEndpoint` was
given in the options, and then never providing one. Passing it along
would probably be the better solution, but there are a lot of places to
get that wrong as things stand right now. It's not clear to me where the
receipt URL really should live--eg., whether it should properly be
part of the connection config.

packages/upload-client/src/receipts.js

@@ -1,6 +1,6 @@
 import retry, { AbortError } from 'p-retry'
 import { CAR } from '@ucanto/transport'
-import { receiptsEndpoint } from './service.js'
+import { receiptsEndpoint as defaultReceiptsEndpoint } from './service.js'
 import { REQUEST_RETRIES } from './constants.js'
 
 export class ReceiptNotFound extends Error {
@@ -77,6 +77,19 @@ export async function poll(taskCid, options = {}) {
   )
 }
 
+/**
+ * Calculate a receipt endpoint from the URL of a channel, if it has one.
+ * @param {import('@ucanto/interface').Channel<Record<string, any>>} channel
+ */
+function receiptEndpointFromChannel(channel) {
+  if ('url' in channel && channel.url instanceof URL) {
+    const url = channel.url
+    return new URL('/receipt/', url.toString())
+  } else {
+    return null
+  }
+}
+
 /**
  * Get a receipt for an executed task by its CID.
  *
@@ -85,11 +98,13 @@ export async function poll(taskCid, options = {}) {
  * @returns {Promise<import('@ucanto/client').Result<import('@ucanto/interface').Receipt, Error>>}
  */
 async function get(taskCid, options = {}) {
+  const channel = options.connection?.channel
+  const receiptsEndpoint =
+    options.receiptsEndpoint ??
+    (channel && receiptEndpointFromChannel(channel)) ??
+    defaultReceiptsEndpoint
   // Fetch receipt from endpoint
-  const url = new URL(
-    taskCid.toString(),
-    options.receiptsEndpoint ?? receiptsEndpoint
-  )
+  const url = new URL(taskCid.toString(), receiptsEndpoint)
   const fetchReceipt = options.fetch ?? globalThis.fetch.bind(globalThis)
   const workflowResponse = await fetchReceipt(url)
   /* c8 ignore start */