Prevent Infinite Recursion on Cyclic Page Corruption

[am-kantox]

During database startup/recovery or integrity checks on a corrupted .redb file, B-tree tree-walking operations could get stuck in an infinite recursion, eventually exhausting the call stack and crashing the application.

This was occurring in two main recursive operations:

1. RawBtree::verify_checksum_helper: Recursing down branch children to verify leaf checksums.
2. UntypedBtree::visit_pages_helper: Recursing down branch children to visit pages.

If page corruption introduces cyclic pointers (e.g., a child page number points to an ancestor page), these functions would loop indefinitely.

This commit resolves the issue by introducing cycle detection:

• In RawBtree::verify_checksum_helper, we track the active DFS traversal path via a visited stack (Vec<PageNumber>). If a page has already been visited in the active path, it shotcircuits to return Ok(false) (checksum verification failure) instead of looping, allowing standard database repair/rollback to recover or fail gracefully.
• In UntypedBtree::visit_pages_helper, we verify if a child page is already part of the active path (path.parents() or current page_number). If a cycle is detected, it returns a StorageError::Corrupted error.

Added unit tests:

• test_page_path_cycle_detection to verify loop detection on PagePaths.
• test_verify_checksum_cycle_detection to verify loop detection on active verify stacks.

I met this issue in the real project.

[codecov]

Codecov Report

❌ Patch coverage is 99.14530% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.29%. Comparing base (adf0353) to head (a03db6c).

Files with missing lines	Patch %	Lines
src/tree_store/btree.rs	99.14%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1301      +/-   ##
==========================================
+ Coverage   90.22%   90.29%   +0.06%     
==========================================
  Files          36       36              
  Lines       16457    16572     +115     
==========================================
+ Hits        14848    14963     +115     
  Misses       1609     1609              

Files with missing lines	Coverage Δ
src/tree_store/btree.rs	88.69% <99.14%> (+1.48%)	⬆️

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