fix(vector): URL-encode DAV paths and unwrap TaskGroup exceptions

Two ingest-robustness fixes from card 309 (OHR-Bench smoke-test triage).

WebDAV paths flowed through the client already URL-decoded (unquote on the
PROPFIND/REPORT <d:href>, or raw MCP-tool input), so a '#' reached httpx as a
URL fragment and silently truncated the request -> spurious 404 on otherwise
valid files (e.g. law filenames with '#', commas, double/trailing spaces).
Route every caller-path builder through a new _webdav_path helper that
percent-encodes the path once (preserving separators); the MOVE/COPY
Destination header is encoded too.

Vector-sync runs inside anyio task groups, so a child-task failure surfaced as
a BaseExceptionGroup whose str() is the useless "unhandled errors in a
TaskGroup (N sub-exception)" -- hiding the real ConnectError operators need.
Add format_exception_group to flatten the group to its leaf exceptions and use
it at the broad catch/log sites in processor.py and oauth_sync.py.

Refs: Deck board 12 card 309 (AC #4 filename handling, AC #2 observability).

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

nextcloud_mcp_server/vector/oauth_sync.py

@@ -30,6 +30,7 @@ from httpx import BasicAuth, HTTPStatusError
 from nextcloud_mcp_server.auth.storage import RefreshTokenStorage
 from nextcloud_mcp_server.client import NextcloudClient
 from nextcloud_mcp_server.config import get_settings
+from nextcloud_mcp_server.vector._errors import format_exception_group
 from nextcloud_mcp_server.vector.processor import process_document
 from nextcloud_mcp_server.vector.queue.ports import TaskProducer
 from nextcloud_mcp_server.vector.scanner import DocumentTask, scan_user_documents
@@ -257,7 +258,7 @@ async def user_scanner_task(
             logger.error(
                 "[BasicAuth] Scanner error for %s: %s (%s/%s)",
                 user_id,
-                e,
+                format_exception_group(e),
                 consecutive_errors,
                 max_consecutive_errors,
                 exc_info=True,
@@ -347,12 +348,15 @@ async def multi_user_processor_task(
                     worker_id,
                     doc_task.doc_type,
                     doc_task.doc_id,
-                    e,
+                    format_exception_group(e),
                     exc_info=True,
                 )
             else:
                 logger.error(
-                    "[BasicAuth] Processor %s error: %s", worker_id, e, exc_info=True
+                    "[BasicAuth] Processor %s error: %s",
+                    worker_id,
+                    format_exception_group(e),
+                    exc_info=True,
                 )
 
         finally:
@@ -481,7 +485,11 @@ async def user_manager_task(
                 logger.info("[BasicAuth] Stopped %s scanner(s)", len(revoked_users))
 
         except Exception as e:
-            logger.error("[BasicAuth] User manager error: %s", e, exc_info=True)
+            logger.error(
+                "[BasicAuth] User manager error: %s",
+                format_exception_group(e),
+                exc_info=True,
+            )
 
         # Sleep until next poll
         try: