Fix downgraded TsFileResource serialization

[jt2594838]

Description

This PR fixes TsFileResource serialization when the in-memory TimeIndex has been degraded to FileTimeIndex.

When a resource file already exists, serialization now preserves the device-level time index stored in the resource file instead of serializing the degraded in-memory FileTimeIndex directly.

Tests

• mvn test -pl iotdb-core/datanode -Dtest=TsFileResourceTest
• mvn verify -DskipUTs '-Dit.test=IoTDBRegionMigrateNormalITForIoTV2BatchIT#migrateRegionWithDegradedTimeIndexTest' -DfailIfNoTests=false '-Dfailsafe.failIfNoSpecifiedTests=false' '-Drat.skip=true' -pl integration-test -am -P with-integration-tests -PClusterIT

Follow-up

After rebasing to the latest master and reverting FileTimeIndex serialization, a local rerun of TsFileResourceTest was blocked by unrelated datanode compile errors in pipe classes.

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 41.89%. Comparing base (e231bbe) to head (99ee9a2).

Files with missing lines	Patch %	Lines
...torageengine/dataregion/tsfile/TsFileResource.java	85.71%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #18155      +/-   ##
============================================
+ Coverage     41.77%   41.89%   +0.12%     
  Complexity      318      318              
============================================
  Files          5297     5297              
  Lines        374233   374235       +2     
  Branches      48314    48313       -1     
============================================
+ Hits         156334   156790     +456     
+ Misses       217899   217445     -454     

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