[alpass163]

Description

Motivation

root.__system is maintained by the database itself. In the open-source
version, external clients could write it, delete it, or create it through
SQL. The name was also matched case-insensitively, so root.__System was
treated as the same internal database even though tree-model paths are
case-sensitive.

This PR makes the exact lower-case name reserved and restricts its writes to
dedicated internal writers. Names that differ only in case are ordinary
databases and stay fully usable.

Changes

• Internal database identification: add InternalDatabaseUtils to
  identify root.__system and the audit databases. Matching is
  case-sensitive and includes descendants (root.__system.x).
• Internal caller flag: add SessionInfo.internal,
  IAuditEntity.isInternal() and TreeAccessCheckContext.internal. Only
  the internal metric reporter (IoTDBInternalLocalReporter) and the
  internal insert RPC (DataNodeInternalRPCServiceImpl.insertRecord) set
  it. AutoCreateSchemaExecutor now passes the query context instead of a
  bare UserEntity, so the flag survives automatic schema creation.
• Write guard: add TreeAccessCheckVisitor.checkSystemDatabaseWrite.
  It is applied to inserts, timeseries DDL, template statements, TTL,
  logical view targets and sources, and centrally in
  checkTimeSeriesPermission.
• View rewrite: AnalyzeUtils and the delete analysis re-check the real
  device paths after a writable view is rewritten, so a view created before
  this restriction cannot write root.__system.
• LOAD path: AccessControlImpl.checkFullPathWriteDataPermission applies
  the same guard.
• Database DDL: visitSetDatabase rejects creating root.__system (and
  its descendants) through SQL; visitAlterDatabase and
  visitDeleteDatabase already rejected altering and deleting it.
• Case-sensitive audit matching: Audit helpers and the direct
  equalsIgnoreCase comparisons in AccessControlImpl,
  ITableAuthCheckerImpl, ClusterConfigTaskExecutor, ConfigManager,
  PartitionInfo and SubscriptionInfo now use equals.
• Example and documentation: the statistics trigger example writes to
  root.statistics instead of root.__system.statistics, and
  iotdb-system.properties.template states that the IOTDB session metric
  reporter is not supported because its writes are rejected.