[core-lead-agent] Closes the regression-test gap on PR #170 (Core-BE's
fix for #159 retry-storm). Original PR shipped the inline conditional
without a unit test; this commit:
1. Extracts the inline `(proxyErr != nil && len(respBody) > 0 && 2xx)`
predicate into a named helper `isDeliveryConfirmedSuccess`. Same
behavior; the call site now reads `if isDeliveryConfirmedSuccess(...)`.
2. Adds `TestIsDeliveryConfirmedSuccess` — 10-case table test covering:
- The new branch (2xx + body + transport error → recover as success):
status=200, status=299, status=200+min-body
- Each precondition failing in isolation:
* nil proxyErr → false (no decision)
* empty/nil body → false (no work to recover)
* 4xx/5xx/3xx body → false (agent-signalled failure or redirect)
* <200 status → false (not 2xx)
Test-pattern mirrors the existing `TestIsTransientProxyError_Retries...`
and `TestIsQueuedProxyResponse` table tests in the same file — same
file-local mock-error pattern, no new test infra.
97768272a3