fix(agent): preserve MoA facade when rebuilding primary client (stream retry, rotation, fallback+restore)

[x1051445024]

Problem

Issue #78382 covers the MoA facade being dropped when agent.client is replaced — but the fix there only covers three call sites (restore_primary_runtime, try_recover_primary_transport, switch_model). The generic rebuild path _replace_primary_openai_client (used by stream-retry pool cleanup, credential rotation, dead-connection cleanup, and fallback+restore) still goes through create_openai_client, which builds a native OpenAI client while agent.provider stays "moa".

Second symptom (this PR's report): with a MOA preset active, when the aggregator returns an empty stream (EmptyStreamError, HTTP 200 / zero valid chunks), the stream-retry machinery calls _replace_primary_openai_client(reason="stream_retry_pool_cleanup"). The rebuilt native client picks up whatever relay base_url happens to be in _client_kwargs at that point, and the retried request — still carrying the MOA preset name as the model — is sent to an unrelated relay gateway, which answers:

HTTP 503: 分组 Codex 下模型 <preset> 无可用渠道（distributor）
{'error': {'code': 'model_not_found', ... 'type': 'new_api_error'}}

No such group exists anywhere in the user's config; the request simply leaked to a foreign gateway. On top of that, the facade is gone for the rest of the turn, so subsequent primary calls keep hitting the wrong endpoint (or raise the _moa_prepared_request TypeError from #78382).

Fix

In create_openai_client, when agent.provider == "moa", return build_moa_facade(agent, model) instead of building a native OpenAI client. Since every rebuild entry point funnels through create_openai_client, this covers _replace_primary_openai_client and any future rebuild path. The three already-fixed call sites assign the facade directly and never reach create_openai_client, so they are unaffected.

build_moa_facade also re-wires the reference relay (see #53802), so display events (moa.reference / moa.aggregating) survive rebuilds too.

Verification

• python -m py_compile passes on the modified file.
• Simulated rebuild: create_openai_client(SimpleNamespace(provider="moa", model=preset, ...), {}, reason="stream_retry_pool_cleanup") returns a MoAClient — facade preserved.
• Real-world (Hermes 0.19.0 runtime, MOA preset = one reference + one aggregator on custom providers): after the patch, aggregator empty-stream retries re-run the proper reference/aggregator fan-out; the foreign-gateway 503 no longer appears.

Notes

• Pure client-factory guard; no config or docs changes.
• Related: MOA: _moa_prepared_request leaked to native OpenAI client when agent.client is replaced (fallback/rotation) #78382 (TypeError symptom of the same root cause), fix(moa): restore virtual runtime after fallback #53802 (reference relay), [Bug]: Streaming reasoning models may incorrectly trigger "Provider returned an empty stream" #56516 / [Bug]: Provider error chunk (HTTP-200 SSE carrying a 400) is misclassified as "empty stream" and retried forever #65631 / Streaming path swallows provider error SSE events (error_type/error_message) as a generic EmptyStream, bypassing context_length_exceeded auto-reduction #68451 (empty-stream misclassification family — this PR does not change retry policy, only where the retried request goes).