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/processor.py

@@ -31,6 +31,7 @@ from nextcloud_mcp_server.observability.tracing import trace_operation
 from nextcloud_mcp_server.search.pdf_highlighter import PDFHighlighter
 from nextcloud_mcp_server.usage import UsageEventStore
 from nextcloud_mcp_server.vector import payload_keys
+from nextcloud_mcp_server.vector._errors import format_exception_group
 from nextcloud_mcp_server.vector.document_chunker import (
     DocumentChunker,
     PageAwareChunker,
@@ -269,7 +270,7 @@ async def processor_task(
                 worker_id,
                 doc_task.doc_type,
                 doc_task.doc_id,
-                e,
+                format_exception_group(e),
                 exc_info=True,
             )
             # Continue to next document (no task_done() needed with streams)
@@ -443,7 +444,7 @@ async def process_document(
                             max_retries,
                             doc_task.doc_type,
                             doc_task.doc_id,
-                            e,
+                            format_exception_group(e),
                             extra={
                                 "doc_id": doc_task.doc_id,
                                 "doc_type": doc_task.doc_type,
@@ -460,7 +461,7 @@ async def process_document(
                             doc_task.doc_type,
                             doc_task.doc_id,
                             max_retries,
-                            e,
+                            format_exception_group(e),
                             extra={
                                 "doc_id": doc_task.doc_id,
                                 "doc_type": doc_task.doc_type,