fix: prevent unnecessary full reindex when Qdrant collection already exists

[roomote-v0]

This PR attempts to address Issue #12145. Feedback and guidance are welcome.

Problem

Users with persistent Qdrant storage were seeing a full reindex every time they reopened VS Code, even though the collection and its data were intact in Qdrant.

Root Causes

1. getCollectionInfo() treated ALL errors as "collection not found" - Connection failures, timeouts, and other transient errors were caught and returned as null, making initialize() think the collection did not exist and create a new one.

2. Aggressive error cleanup destroyed valid data - The orchestrator error handler called clearCollection() + clearCacheFile() on any indexing error, even transient ones on an existing collection with valid data.

3. hasIndexedData() returned false on connection errors - Same issue as Add options to always approve write and execute operations #1, causing the orchestrator to skip the incremental scan path and do a full scan.

Changes

src/services/code-index/vector-store/qdrant-client.ts

• Added isNotFoundError() helper to distinguish 404 (collection missing) from other errors
• getCollectionInfo() now returns null only for 404 errors; propagates other errors
• hasIndexedData() lets connection errors propagate instead of silently returning false
• collectionExists() inherits the same error propagation

src/services/code-index/orchestrator.ts

• Error handler now checks collectionCreated flag to decide cleanup behavior
• New collection (just created): clear collection + cache as before
• Existing collection (pre-existing data): flush/persist cache instead of clearing, skip clearCollection() to preserve the user's existing index
• Connection failures before indexing starts: preserve cache for future incremental scan (unchanged)

Tests updated

• qdrant-client.spec.ts: Non-404 errors now throw instead of creating a collection
• orchestrator.spec.ts: Split error-cleanup test into two cases (new vs existing collection)

Fixes #12145

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