Fix data availability checker race condition causing partial data columns to be served over RPC (#7961)

Partially resolves #6439, an simpler alternative to #7931.

Race condition occurs when RPC data columns arrives after a block has been imported and removed from the DA checker:
1. Block becomes available via gossip
2. RPC columns arrive and pass fork choice check (block hasn't been imported)
3. Block import completes (removing block from DA checker)
4. RPC data columns finish verification and get imported into DA checker

This causes two issues:
1. **Partial data serving**: Already imported components get re-inserted, potentially causing LH to serve incomplete data
2. **State cache misses**: Leads to state reconstruction, holding the availability cache write lock longer and increasing race likelihood

### Proposed Changes

1. Never manually remove pending components from DA checker. Components are only removed via LRU eviction as finality advances. This makes sure we don't run into the issue described above.
2. Use `get` instead of `pop` when recovering the executed block, this prevents cache misses in race condition. This should reduce the likelihood of the race condition
3. Refactor DA checker to drop write lock as soon as components are added. This should also reduce the likelihood of the race condition

**Trade-offs:**

This solution eliminates a few nasty race conditions while allowing simplicity, with the cost of allowing block re-import (already existing).

The increase in memory in DA checker can be partially offset by a reduction in block cache size if this really comes an issue (as we now serve recent blocks from DA checker).

beacon_node/beacon_chain/src/data_availability_checker.rs

@@ -38,19 +38,18 @@ use crate::observed_data_sidecars::ObservationStrategy;
 pub use error::{Error as AvailabilityCheckError, ErrorCategory as AvailabilityCheckErrorCategory};
 use types::non_zero_usize::new_non_zero_usize;
 
-/// The LRU Cache stores `PendingComponents`, which can store up to `MAX_BLOBS_PER_BLOCK` blobs each.
+/// The LRU Cache stores `PendingComponents`, which store block and its associated blob data:
 ///
 /// * Deneb blobs are 128 kb each and are stored in the form of `BlobSidecar`.
 /// * From Fulu (PeerDAS), blobs are erasure-coded and are 256 kb each, stored in the form of 128 `DataColumnSidecar`s.
 ///
 /// With `MAX_BLOBS_PER_BLOCK` = 48 (expected in the next year), the maximum size of data columns
-/// in `PendingComponents` is ~12.29 MB. Setting this to 64 means the maximum size of the cache is
-/// approximately 0.8 GB.
+/// in `PendingComponents` is ~12.29 MB. Setting this to 32 means the maximum size of the cache is
+/// approximately 0.4 GB.
 ///
-/// Under normal conditions, the cache should only store the current pending block, but could
-///  occasionally spike to 2-4 for various reasons e.g. components arriving late, but would very
-/// rarely go above this, unless there are many concurrent forks.
-pub const OVERFLOW_LRU_CAPACITY: NonZeroUsize = new_non_zero_usize(64);
+/// `PendingComponents` are now never removed from the cache manually are only removed via LRU
+/// eviction to prevent race conditions (#7961), so we expect this cache to be full all the time.
+pub const OVERFLOW_LRU_CAPACITY: NonZeroUsize = new_non_zero_usize(32);
 pub const STATE_LRU_CAPACITY_NON_ZERO: NonZeroUsize = new_non_zero_usize(32);
 pub const STATE_LRU_CAPACITY: usize = STATE_LRU_CAPACITY_NON_ZERO.get();
 
@@ -346,11 +345,6 @@ impl<T: BeaconChainTypes> DataAvailabilityChecker<T> {
             .put_pending_executed_block(executed_block)
     }
 
-    pub fn remove_pending_components(&self, block_root: Hash256) {
-        self.availability_cache
-            .remove_pending_components(block_root)
-    }
-
     /// Verifies kzg commitments for an RpcBlock, returns a `MaybeAvailableBlock` that may
     /// include the fully available block.
     ///
@@ -589,8 +583,8 @@ impl<T: BeaconChainTypes> DataAvailabilityChecker<T> {
 
         // Check indices from cache again to make sure we don't publish components we've already received.
         let Some(existing_column_indices) = self.cached_data_column_indexes(block_root) else {
-            return Ok(DataColumnReconstructionResult::RecoveredColumnsNotImported(
-                "block already imported",
+            return Err(AvailabilityCheckError::Unexpected(
+                "block no longer exists in the data availability checker".to_string(),
             ));
         };