harness(openai): surface the full transport error source chain

[senamakel]

Summary

send_checked mapped a reqwest send failure with a plain {e}, which renders only the outermost context (error sending request for url (...)). The actionable cause — connection refused (os error 111), a TLS failure, a DNS error — lives one or more links down the source() chain and was dropped.

Add error_source_chain(err) which walks the full source() chain joined by ": ", and use it when mapping the transport failure. This lets a downstream host classify on the TinyAgentsError::Model message (loopback-offline detection, transport retryability) — parity with what a host that kept the raw reqwest::Error chain could do.

Commands run

• cargo fmt --check
• cargo test --lib openai:: — 62 pass
• cargo clippy --lib -- -D warnings — clean

Behavior

Additive; the only change to existing output is a richer transport-error message (now includes the cause chain).

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📥 Commits

Reviewing files that changed from the base of the PR and between 7c6e81a and b818378.

📒 Files selected for processing (3)

• src/harness/providers/openai/mod.rs
• src/harness/providers/openai/test.rs
• src/harness/providers/openai/transport.rs

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