[Candidate / unverified] Solution for 63bb2cff-875b-47ff-9729-e80a9119ff4b: Read the embedded HTTP status from the error message. For 407, supply valid prox

## Support semantics This is a **candidate / unverified** Solution from the KFA Solution Factory Tier-2 gate. It is source-grounded research material, not an Outcome, and does **not** claim independent reproduction or confirmed field success. ## Cause CONNECT tunnel setup requires HTTP 200 from the proxy; other statuses abort the request. Proxy connection loss is treated as non-recoverable on the same socket since v8.10.1. ## Steps 1. Log the full `error.cause` chain including undici `code` and message. 2. If message matches `Proxy response (407) !== 200 when HTTP Tunneling`, configure proxy authentication on the agent/proxy URL. 3. If message shows 403 or other policy codes, adjust corporate proxy allowlists rather than retrying fetch. 4. If connections hang without failing, check undici version and upgrade to >=8.10.1 for `ProxyConnectionError` fail-fast behavior. 5. For TLS errors to an HTTPS proxy, validate CA/trust store (`UND_ERR_PRX_TLS`). ## Expected result Proxy auth/policy failures surface immediately with interpretable status codes; upgraded undici stops infinite retry when the proxy connection is torn down. ## Misleading approaches - Retrying fetch in a loop on 407 without supplying proxy credentials. - Assuming `UND_ERR_ABORTED` always means the origin server aborted the request. - Expecting undici to automatically prompt for proxy credentials interactively. ## Limitations - Upgrade to 8.10.1 changes failure mode (fail-fast) but does not make 407 succeed without credentials. - Which Node.js release bundles undici >=8.10.1 was not verified here. - Not reproduced in this worker run. ## Unknowns - Exact proxy product returning a given status in the caller's environment. - Whether EnvHttpProxyAgent rewrites 407 handling identically to ProxyAgent for a given undici release. ## Evidence claims - (documented_platform_behavior) Non-200 CONNECT responses produce RequestAbortedError with message template Proxy response (${statusCode}) !== 200 when HTTP Tunneling. — https://github.com/nodejs/undici/blob/main/lib/dispatcher/proxy-agent.js - (released_fix) ProxyConnectionError (UND_ERR_PRX_CONN) was added in undici 8.10.1 to fail instead of retrying forever when the proxy connection is torn down. — https://github.com/nodejs/undici/blob/main/docs/docs/api/Errors.md - (documented_platform_behavior) Default messages for proxy connection failures include 'Proxy Connection failed' and 'Secure Proxy Connection failed'. — https://github.com/nodejs/undici/blob/main/lib/core/errors.js