Add ReadOnly status reason and StatusReason display

[d-wang-commit]

Classify ReadOnly trigger points with priority-ordered reasons (Stopping > Manual > UnrecoverableError > DiskFull > unclassified), propagate the reason through heartbeats as a separate statusReason field, and show it as a dedicated StatusReason column in SHOW CLUSTER / SHOW CONFIGNODES / SHOW DATANODES and the information_schema.NODES view instead of the merged Status(reason) string. UnrecoverableError reasons carry the truncated exception message with a class-name fallback.

Description

Classifies ReadOnly trigger points with reasons and displays the reason in a dedicated column instead of merging
it into the Status string.

Reason state machine (CommonConfig)

• ReadOnly entries carry stable tokens with priority Stopping > Manual > UnrecoverableError > DiskFull, plus a
  legacy unclassified (null) reason at the lowest priority.
• Same-status writes never downgrade the reason (a DiskFull alarm cannot overwrite a Manual ReadOnly); a
  same-priority UnrecoverableError keeps the first reason. Status and reason are written atomically.

Trigger point classification

• Disk-full sampling / folder exhaustion → DiskFull
• SET SYSTEM TO READONLY (local or cluster-wide) → Manual
• Shutdown flush phase → Stopping
• Storage / WAL / checkpoint unrecoverable errors → UnrecoverableError, <timestamp>, <message>; the message is
  truncated to 256 chars with ... and falls back to the exception class name when empty

Reason propagation & display

• DataNode heartbeats carry statusReason alongside status, read from one snapshot to avoid torn pairs.
• SHOW CLUSTER / SHOW CLUSTER DETAILS / SHOW CONFIGNODES / SHOW DATANODES insert a StatusReason column
  right after Status only when at least one node has a reason; nodes without one show NULL, and the Status
  column keeps the plain status (no more merged ReadOnly(DiskFull) strings).
• information_schema.NODES gains a status_reason column.
• New thrift fields are optional: responses from an old ConfigNode (field unset) keep the legacy column layout.

Tests

• UT: CommonConfigTest (priority chain, first-UnrecoverableError-wins, unclassified-reason semantics, truncation / class-name fallback, same-status reason preservation); DataNodeInternalRPCServiceImplDiskTest (auto-recovery only for an exact
  ReadOnly+DiskFull state); DataNodeInternalRPCServiceImplSetSystemStatusTest (Manual classification); Show*TaskTest (dynamic column layouts incl. old-ConfigNode fallback); NodeCacheTest (reason propagation; a failure-detector Unknown never carries a reason);
  FolderManagerTest; DataNodeShutdownHookTest.
• IT: IoTDBSetSystemStatusTableIT (end-to-end Manual reason through SHOW DATANODES, plain status in the NODES view); IoTDBDatabaseIT (NODES schema).

Negative effects

• SHOW statements gain one extra column only when a reason exists; clients parsing by column name are unaffected.
• Each reason is bounded to 256 chars and only travels in heartbeats / SHOW responses, so the payload overhead is negligible.

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This PR has:

• been self-reviewed.
  • concurrent read
  • concurrent write
  • concurrent read and write
• added documentation for new or modified features or behaviors.
• added Javadocs for most classes and all non-trivial methods.
• added or updated version, license, or notice information
• added comments explaining the "why" and the intent of the code wherever would not be obvious
  for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold
  for code coverage.
• added integration tests.
• been tested in a test IoTDB cluster.

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Key changed/added classes (or packages if there are too many classes) in this PR