fix(provider-utils): retry on Node.js socket termination errors

[sidx1024]

Summary

TypeError: terminated and SocketError: other side closed are thrown by Node.js undici when the TCP socket dies mid-response (e.g. during long-running generateText calls with reasoning models). These are transient network errors, but handleFetchError only recognizes "fetch failed" / "failed to fetch", so they fall through as raw errors and are not retried by the exponential backoff logic.

This adds detection for socket termination errors (terminated, other side closed, UND_ERR_SOCKET code) so they are properly wrapped as retryable APICallErrors.

Reproduction

When using generateText with models that have long response times (e.g. gpt-5.2 with reasoningEffort: "high"), the TCP socket can be terminated by intermediate infrastructure. The error surfaces as:

AI_APICallError: Failed to process successful response
  cause: TypeError: terminated
    at Fetch.onAborted (node:internal/deps/undici/undici:12826:53)

or:

SocketError: other side closed
  code: 'UND_ERR_SOCKET'
    at TLSSocket.onHttpSocketEnd (node:internal/deps/undici/undici:6294:28)

Neither error is retried because handleFetchError returns them as-is.

Changes

• Added isSocketError() helper that matches terminated, other side closed messages and UND_ERR_SOCKET error code
• Socket errors are now wrapped as APICallError with isRetryable: true
• Added 3 test cases covering both error shapes

Test plan

• TypeError: terminated → retryable APICallError
• SocketError: other side closed (code UND_ERR_SOCKET) → retryable APICallError
• TypeError: terminated as cause of fetch failed → retryable (existing path, confirmed)
• All existing tests pass

Fixes #11265
Fixes #9775