[STORAGE] Send diffs from PinnedWritePage to Wal

[kTrzcinskii]

Updated cache to allow dropping multiple pages as one MultiPageOperation to wal.
If PinnedWritePage has diffs during its drop then they are sent as SinglePageOperation to wal.

[Copilot]

Pull request overview

This PR implements automatic and manual WAL (Write-Ahead Log) logging for page modifications in the storage cache. When a PinnedWritePage is dropped, its diffs are automatically sent to the WAL as a single-page operation. Alternatively, multiple pages can be manually dropped together using Cache::drop_write_pages() to create an atomic multi-page WAL operation.

Key changes:

• Added automatic WAL logging on PinnedWritePage drop for pages with modifications
• Implemented Cache::drop_write_pages() method to batch multiple page updates into a single atomic WAL operation
• Changed wal_client in Cache from Option<WalClient> to Option<Arc<WalClient>> to enable sharing across PinnedWritePage instances

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 7 comments.

File	Description
storage/src/page_diff.rs	Added empty() method to check if a PageDiff has any diffs (used to avoid logging empty changes)
storage/src/cache.rs	Added wal field to PinnedWritePage, implemented automatic WAL logging in Drop, added drop_write_pages() method for batching, changed wal_client to use Arc, and added comprehensive tests for all WAL logging scenarios

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