Upgrade async-openai from our own etkecc fork to upstream's 0.31.1

Switches `async-openai` from our own etkecc fork (0.28.1-patched) to the
official crates.io version 0.31.1.

We adapt to async-openai's types reorganization and making use of crate
features to only enable what we need.

Cargo.lock

@@ -191,8 +191,9 @@ dependencies = [
 
 [[package]]
 name = "async-openai"
-version = "0.28.1"
-source = "git+https://github.com/etkecc/async-openai?branch=async-openai-v0.28.1-patched#856953c2d4485342df625fd0525363362075e8a8"
+version = "0.31.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1cc4c602409022b854d89332fae01a6c69a4122fdd0ec66a071b9b5c3a87750b"
 dependencies = [
  "async-openai-macros",
  "backoff",
@@ -201,23 +202,26 @@ dependencies = [
  "derive_builder 0.20.2",
  "eventsource-stream",
  "futures",
- "rand 0.8.5",
+ "rand 0.9.2",
  "reqwest 0.12.24",
  "reqwest-eventsource 0.6.0",
  "secrecy",
  "serde",
  "serde_json",
+ "serde_urlencoded",
  "thiserror 2.0.17",
  "tokio",
  "tokio-stream",
  "tokio-util",
  "tracing",
+ "url",
 ]
 
 [[package]]
 name = "async-openai-macros"
 version = "0.1.0"
-source = "git+https://github.com/etkecc/async-openai?branch=async-openai-v0.28.1-patched#856953c2d4485342df625fd0525363362075e8a8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0289cba6d5143bfe8251d57b4a8cac036adf158525a76533a7082ba65ec76398"
 dependencies = [
  "proc-macro2",
  "quote",