HBASE-29863 Adding support for capturing files read during scan (branch-2.6 backport)

[mnpoonia]

Summary

Backport of HBASE-29863 to branch-2.6:

• #7823 — HBASE-29863 Adding support for capturing files read during scan
• #8177 — HBASE-29863 Adding default implementation for getFilesRead API (Addendum)

Adaptations for branch-2.6

• ExtendedCell → Cell throughout scanner interfaces and test helpers (branch-2.6 removed ExtendedCell)
• Migrated new test files and test methods from JUnit 4 to JUnit 5 (branch-2.6 enforces JUnit 5 via banned-imports)
• Replaced TestName rule with TestInfo injection for method name access
• Fixed JUnit 4-style assertion parameter order ("message", value) to JUnit 5 order (value, "message") in all new test code

Test plan

• mvn install -DskipTests -Dhadoop.profile=3.0 compiles cleanly (verified locally)
• TestKeyValueHeap#testGetFilesRead
• TestStoreScanner#testGetFilesRead
• TestSwitchToStreamRead#testGetFilesReadAfterStreamSwitch
• TestClientSideRegionScanner#testGetFilesRead
• TestHRegion#testRegionScannerGetFilesRead
• TestHMobStore#testMobStoreScannerGetFilesRead
• TestTableSnapshotInputFormat#testTableSnapshotRegionRecordReaderGetFilesRead