fix(mcp): allow clear_index to drop orphaned collections without local path

[MukundaKatta]

What

clear_index validated that the codebase path exists on disk before dropping its Milvus collection:

if (!fs.existsSync(absolutePath)) { ... "does not exist" ... }
const stat = fs.statSync(absolutePath);
if (!stat.isDirectory()) { ... "is not a directory" ... }

But the path is only used as the collection identifier. If a codebase is moved or deleted on disk, its collection is left orphaned with no way to clear it, since the existence check fails before any Milvus call runs.

Change

Remove the fs.existsSync and fs.statSync directory checks from handleClearIndex. The existing isIndexed/isIndexing check already proves there is a collection to drop, and the path serves only as the key. Index and search handlers, which genuinely need a local directory, are untouched.

Testing

• tsc --noEmit clean for the mcp package
• pnpm --filter @zilliz/claude-context-mcp test passes (4/4)
• core package builds

Fixes #375