Update async-openai to v0.38.0

Closes #137. The 0.36 -> 0.38 bump (skipping 0.37) introduces Tower-based
middleware support and a fix to the ReasoningItem `type` field, but
neither affects baibot's call sites — no code adaptation was needed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -184,12 +184,11 @@ checksum = "4288f83726785267c6f2ef073a3d83dc3f9b81464e9f99898240cced85fce35a"
 
 [[package]]
 name = "async-openai"
-version = "0.36.0"
+version = "0.38.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "78efd86812da6b76e2be727c7015522dde20495855390496099f062b01d2c868"
+checksum = "322adbd4772f41a55fd30c10b924a30cf557ce65bbe7ebacda5a068404645604"
 dependencies = [
  "async-openai-macros",
- "backoff",
  "base64 0.22.1",
  "bytes",
  "derive_builder 0.20.2",
@@ -206,15 +205,16 @@ dependencies = [
  "tokio",
  "tokio-stream",
  "tokio-util",
+ "tower",
  "tracing",
  "url",
 ]
 
 [[package]]
 name = "async-openai-macros"
-version = "0.1.1"
+version = "0.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "81872a8e595e8ceceab71c6ba1f9078e313b452a1e31934e6763ef5d308705e4"
+checksum = "492a944774207eed3acf425214eadbd6ce84a2b89331164ff1c11bae92b26302"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -4339,8 +4339,10 @@ dependencies = [
  "pin-project-lite",
  "sync_wrapper 1.0.2",
  "tokio",
+ "tokio-util",
  "tower-layer",
  "tower-service",
+ "tracing",
 ]
 
 [[package]]