Fix load Sonar warnings

[Caideyipi]

Description

This PR fixes Sonar warnings in load-related code by:

• replacing a Map get/null/put pattern with computeIfAbsent in load routing.
• checking Optional presence before accessing TsFileResource start/end times.
• reusing a guarded ActiveLoadEntry value instead of repeated Optional.get() calls.

---

This PR has:

• been self-reviewed.
• 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.

---

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

• LoadSingleTsFileNode
• LoadTsFileScheduler
• ActiveLoadTsFileLoader

Tests

• mvn '-Ddevelocity.off=true' spotless:check -pl iotdb-core/datanode
• git diff --check

[sonarqubecloud]

Quality Gate passed

Issues
0 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 0% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 40.66%. Comparing base (a437cd4) to head (c96ee4d).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...n/planner/plan/node/load/LoadSingleTsFileNode.java	0.00%	8 Missing ⚠️
...rageengine/load/active/ActiveLoadTsFileLoader.java	0.00%	8 Missing ⚠️
...ngine/plan/scheduler/load/LoadTsFileScheduler.java	0.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #17882      +/-   ##
============================================
- Coverage     40.67%   40.66%   -0.02%     
- Complexity     2620     2621       +1     
============================================
  Files          5244     5244              
  Lines        362349   362353       +4     
  Branches      46650    46651       +1     
============================================
- Hits         147385   147347      -38     
- Misses       214964   215006      +42     

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