fix(vector): retry transient embed errors so a pod rollover drops 0 docs

From card 309 (OHR-Bench smoke-test triage): during a backend-pod rollover the
embedding endpoint was briefly unreachable, and openai.APIConnectionError /
ConnectError propagated unretried (the provider only retried 429). Documents
exhausted the 3 in-process retries and were dropped for that scan cycle.

Broaden the provider-level retry to the transient set -- APIConnectionError,
APITimeoutError, 429, and 5xx -- on the existing exponential backoff (2s->60s,
5 attempts), so a few seconds of retry rides through the rollover. Permanent
4xx (auth, bad request) still re-raise immediately. Generalize the shared
_retry helper (retry_on_rate_limit -> retry_on_transient, predicate renamed to
should_retry, accurate log label) with a back-compat alias; Mistral gets 429+5xx
for parity. The production gateway path inherits this via GatewayProvider, which
delegates to the decorated OpenAIProvider methods.

Add astrolabe_vector_ingest_dropped_total{reason}, incremented when a document
exhausts retries, classified (connection|timeout|rate_limit|server|qdrant|other)
by _drop_reason so the embed-drop rate is alertable per cause. Dropped docs are
NOT marked failed, so the next full scan re-picks them (re-queue via scan loop).

Refs: Deck board 12 card 309 (AC #1 no permanently-dropped docs; embed-drop
metric for AC #5).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/providers/mistral.py

@@ -13,7 +13,7 @@ import logging
 from mistralai.client import Mistral
 from mistralai.client.errors import SDKError
 
-from ._retry import retry_on_rate_limit
+from ._retry import retry_on_transient
 from .base import Provider
 
 logger = logging.getLogger(__name__)
@@ -30,13 +30,17 @@ BATCH_SIZE = 64
 _NO_EMBEDDING_MODEL_MSG = "Embedding not supported - no embedding_model configured"
 
 
-def _is_rate_limit(exc: BaseException) -> bool:
-    """True only for HTTP 429 SDKErrors."""
-    return getattr(exc, "status_code", None) == 429
+def _is_transient(exc: BaseException) -> bool:
+    """Retry HTTP 429 (rate limit) and 5xx (server/transient) SDKErrors."""
+    status = getattr(exc, "status_code", None)
+    return status == 429 or (isinstance(status, int) and status >= 500)
 
 
-_retry_429 = retry_on_rate_limit(
-    SDKError, is_rate_limit=_is_rate_limit, provider_name="Mistral"
+_retry_transient = retry_on_transient(
+    SDKError,
+    should_retry=_is_transient,
+    provider_name="Mistral",
+    label="transient error",
 )
 
 
@@ -90,7 +94,7 @@ class MistralProvider(Provider):
     def supports_generation(self) -> bool:
         return False
 
-    @_retry_429
+    @_retry_transient
     async def embed(self, text: str) -> list[float]:
         """Generate an embedding for a single text."""
         if not self.supports_embeddings:
@@ -169,7 +173,7 @@ class MistralProvider(Provider):
 
         return all_embeddings, total_tokens
 
-    @_retry_429
+    @_retry_transient
     async def _embed_batch_request(
         self, batch: list[str]
     ) -> tuple[list[list[float]], int]: