storage: merge reclaimed tail pages with deserialized FSM state

[adsharma]

Fixes: #400

Summary

Fix recovery-time free-space manager (FSM) inconsistency where tail reclamation after checkpoint deserialization
could leave overlapping free entries, potentially enabling double allocation of the same page after reopen.

Problem

During startup, we:

1. deserialize FSM entries from checkpoint metadata, then
2. reclaim post-checkpoint tail pages via PageManager::reclaimTailPagesIfNeeded().

Previously, tail reclaim used evictAndAddFreePages(), which inserts directly into freeLists and does not merge
overlaps with existing deserialized entries (it only avoids exact duplicate (start,numPages) tuples).
This could produce overlapping free ranges in reopened state.

Root cause

reclaimTailPagesIfNeeded() bypassed the merge/coalescing path used elsewhere by FSM.

Fix

1) Reclaim tail through merge path

In src/storage/page_manager.cpp:

• replaced:
  • freeSpaceManager->evictAndAddFreePages(fileHandle, tail);
• with:
  • freeSpaceManager->addUncheckpointedFreePages(tail);
  • freeSpaceManager->mergeFreePages(fileHandle);

This guarantees tail reclaim is merged with deserialized FSM state (overlap + adjacency coalescing).

2) Explicitly clear moved uncheckpointed list

In src/storage/free_space_manager.cpp:

• after mergePageRanges(std::move(uncheckpointedFreePageRanges), ...), explicitly call:
  • uncheckpointedFreePageRanges.clear();

This mirrors finalizeCheckpoint() behavior and avoids relying on moved-from container state.

Test

Added regression test in test/transaction/checkpoint_test.cpp:

• ReviewFixesTest.ReclaimTailMergesWithDeserializedFSMEntries

Test flow:

• create checkpointed DB state,
• inject a synthetic free range beyond EOF,
• checkpoint again,
• append raw pages to force reclaim-tail-on-reopen,
• reopen and query CALL FSM_INFO(),
• assert recovered free ranges are non-overlapping.

Behavior:

• pre-fix: overlapping entries observed.
• post-fix: no overlaps.

Validation

• make test-build-release
• build/release/test/transaction/transaction_test
  --gtest_filter="ReviewFixesTest.ReclaimTailMergesWithDeserializedFSMEntries"
• sanity: ... --gtest_filter="ReviewFixesTest.HashIndexBasicRecoveryAfterCheckpoint"

All passed.