docs(rust): sync async-openai version pin to 0.38

[amavashev]

What

Follow-up to PR #659 (already merged). The companion cycles-client-rust PR (#37) had to bump async-openai 0.30 → 0.38 to clear `cargo audit --deny warnings` — 0.30.x pulled in two unmaintained transitive deps:

• `backoff 0.4.0` (RUSTSEC-2025-0012)
• `instant 0.1.13` (RUSTSEC-2024-0384, via backoff)

async-openai 0.31+ replaced the retry stack with `tower` and dropped both. The example in cycles-client-rust now pins `0.38`. This PR brings the docs in sync so the how-to matches what an evaluator will actually `cargo add` against.

Changes

`how-to/integrating-cycles-with-async-openai.md`:

• Cargo.toml snippet: `async-openai = "0.30"` → `async-openai = { version = "0.38", default-features = false, features = ["chat-completion", "rustls"] }`. The 0.31+ split exposed per-API features; `chat-completion` is what makes `Client` and the chat-completion types available.
• All three code blocks (basic, streaming, error-handling): import path `async_openai::types::` → `async_openai::types::chat::`. The chat-completion types moved out of the top-level `types::` module in 0.31.
• Version commentary (the paragraph that said "type paths accurate to the 0.30.x line at publication"): replaced with an explanation of the 0.30 → 0.31+ transition (feature gating + `types::chat::` path move + tower-vs-backoff retry stack swap), so readers upgrading from 0.30 understand what changed.
• Dropped the unused `Role` import from the basic example.

Why this is a separate PR

The version-sync commit was pushed to the old PR #659 branch after #659 had already been merged. Rather than reopen the merged PR, branching off main is the clean path.

Test plan

• `npm run build` succeeds
• The rendered page shows the new Cargo.toml feature flags + the `types::chat::` imports
• No broken-link warnings (only Cargo.toml text and import paths changed; no link targets touched)