[Fix] Keep heartbeat alive when metadata lease is fenced

[Caideyipi]

Description

• build schema quota count maps on snapshots and publish them to the heartbeat response only after both device and series counts succeed
• catch MetadataLeaseFencedException while collecting schema counts so the DataNode heartbeat can still complete
• omit both region device and series usage maps after this failure, preventing ConfigNode from applying partial schema count updates
• emit a localized warning and add regression coverage for both method-level atomicity and the fenced-heartbeat path

Tests

• mvn -nsu test -pl iotdb-core/datanode -Dtest=DataNodeInternalRPCServiceImplTest#testHeartbeatSkipsSchemaUsageWhenMetadataLeaseIsFenced
• mvn -o -nsu test-compile -pl iotdb-core/datanode -P with-zh-locale -DskipTests

[sonarqubecloud]

Quality Gate failed

Failed conditions
C Reliability Rating on New Code (required ≥ A)

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[codecov]

Codecov Report

❌ Patch coverage is 60.60606% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.43%. Comparing base (4861027) to head (48b5374).
⚠️ Report is 11 commits behind head on master.

Files with missing lines	Patch %	Lines
...ol/thrift/impl/DataNodeInternalRPCServiceImpl.java	47.05%	9 Missing ⚠️
...org/apache/iotdb/db/schemaengine/SchemaEngine.java	75.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #18343      +/-   ##
============================================
+ Coverage     43.14%   43.43%   +0.29%     
  Complexity      374      374              
============================================
  Files          5364     5366       +2     
  Lines        382623   382978     +355     
  Branches      49749    49815      +66     
============================================
+ Hits         165077   166347    +1270     
+ Misses       217546   216631     -915     

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