fix: set default User-Agent on OpenAI driver HTTP client

[ozekimasaki]

Summary

• reqwest::Client::new() does not set a User-Agent header by default
• Some OpenAI-compatible API providers and proxies reject requests without User-Agent, returning 405 errors
• This sets OpenFang/1.0 as a sensible default, with a graceful fallback to reqwest::Client::new() if the builder fails

Changes

File: crates/openfang-runtime/src/drivers/openai.rs (1 file, +4 −1)

// Before
client: reqwest::Client::new(),

// After
client: reqwest::Client::builder()
    .user_agent("OpenFang/1.0")
    .build()
    .unwrap_or_else(|_| reqwest::Client::new()),

Test plan

• Existing tests pass (no tests assert on User-Agent)
• Non-breaking, backward compatible — only adds a header that was previously absent
• Verified against providers that require User-Agent

[zxl-coder-ai]

The openai adapter already has a with_extra_headers method, it's just not being called during registration. Wouldn't it be more elegant if your PR supported passing the User-Agent via configuration instead?

[ozekimasaki]

Thanks for the feedback!

I considered that, but I think a sensible default User-Agent belongs at the client level rather than in configuration:

• Every HTTP client should identify itself — this is basic HTTP hygiene, not a user-facing setting. reqwest::Client::new() sending no User-Agent at all is the gap this fixes.
• Configuration adds friction — if User-Agent requires config, users who don't set it will hit the same 405 errors silently. A default that "just works" is safer.
• with_extra_headers serves a different purpose — it's designed for per-deployment auth headers (e.g. Copilot IDE tokens), not for setting a baseline client identity.

The two aren't mutually exclusive — users can still override User-Agent via with_extra_headers if needed. This PR just ensures there's always a reasonable default in place.