Revert #50 (transport error source chain) — keep loopback errors classifiable as retryable

[senamakel]

Reverts #50 (error_source_chain). Surfacing the full reqwest cause chain (incl. connection refused (os error N)) caused OpenHuman's host classifier to treat an offline local provider as unreachable/non-retryable — but the host now deliberately keeps a short loopback send error retryable (a local server may be transiently starting up). The host had to pin before #50 to keep that behavior; this revert lets the host pin at a commit that still carries #49 (capability toggles) + #51 (Responses API) without the errno-surfacing. #50 can be reintroduced later behind an opt-in once the host classifier is reconciled.

cargo test --lib openai:: (65 pass), cargo clippy --lib -- -D warnings clean.

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Reviewing files that changed from the base of the PR and between 94a3e97 and a7a7f22.

📒 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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