agent: stabilize AtlasCloud guarded delegate conformance

Root cause: - Scheduled Harness (E2E) run https://github.com/micro/go-micro/actions/runs/28834153403 failed on master at c88a090d10521da2286fc04f8d269660f486868f. - The only failed job was `Provider harnesses (live LLM conformance)`; `Harnesses (mock LLM)` passed. - In the provider conformance job, all providers except AtlasCloud were skipped because no API key was configured. AtlasCloud (`ATLASCLOUD_MODEL=minimaxai/minimax-m3`) reached `atlascloud / agent (model call + tool call)` and exhausted 3 conformance attempts with `missing guarded delegate`. - The live model completed calls but did not exercise the guarded `delegate` tool path required by `TestAgentProviderConformanceMatrix`; later AtlasCloud harnesses (`universe`, `agent-flow`, `plan-delegate`, `a2a-stream-fallback`) passed. This is not a network timeout/rate limit/provider outage log signature. - This appears to be a recurrence of the closed guarded-delegate reliability gap (for example #3917/#4191), but I did not find an open scoped issue tracking the recurrence. Where: - `agent` provider conformance matrix, AtlasCloud/minimax-m3 path. - Failed log line: `conformance_test.go:43: Ask: provider conformance incomplete after 3 attempts: missing guarded delegate`. Acceptance criteria: - Add or tighten deterministic coverage showing the AtlasCloud conformance prompt/retry path requires and reliably elicits the guarded delegate call before passing. - Update the live provider conformance harness/prompt/retry logic so AtlasCloud/minimax-m3 completes the guarded delegate requirement without relying on accidental model behavior. - Verify with `go test ./agent -run TestAgentProviderConformance -count=1` or a narrower relevant regression test, plus the scheduled Provider harness (live LLM conformance) on the next run.