Replace explicit type conversion with instanceof pattern variable in server module

[gaobinlong]

Description

Similar to #19714.

Use the modern instanceof pattern variables instead of explicit type conversions in the server module, this can improve the code readability and reduce potential ClassCastException risks.

Most code are generated by LLM, and I checked them manually and reverted the unnecessary and wrong changes.

Related Issues

No issue, just mentioned this in a previous PR.

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[github-actions]

❌ Gradle check result for d383827: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❌ Gradle check result for 8120806: null

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

✅ Gradle check result for 8120806: SUCCESS

[codecov]

Codecov Report

❌ Patch coverage is 46.66667% with 136 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.18%. Comparing base (c0a32d4) to head (05a24d0).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...g/opensearch/common/inject/internal/MoreTypes.java	16.12%	41 Missing and 11 partials ⚠️
.../org/opensearch/common/inject/internal/Errors.java	0.00%	13 Missing ⚠️
...ucene/search/vectorhighlight/CustomFieldQuery.java	41.17%	4 Missing and 6 partials ⚠️
...a/org/opensearch/extensions/ExtensionsManager.java	0.00%	6 Missing ⚠️
...java/org/opensearch/ingest/ConfigurationUtils.java	64.70%	3 Missing and 3 partials ⚠️
...org/opensearch/ingest/TrackingResultProcessor.java	58.33%	2 Missing and 3 partials ⚠️
...org/opensearch/common/xcontent/XContentHelper.java	0.00%	0 Missing and 3 partials ⚠️
...va/org/opensearch/ingest/ConditionalProcessor.java	25.00%	1 Missing and 2 partials ⚠️
...ain/java/org/opensearch/ingest/IngestDocument.java	87.50%	2 Missing and 1 partial ⚠️
.../main/java/org/opensearch/rest/RestController.java	50.00%	2 Missing and 1 partial ⚠️
... and 22 more

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #19938      +/-   ##
============================================
- Coverage     73.30%   73.18%   -0.13%     
+ Complexity    71582    71519      -63     
============================================
  Files          5785     5785              
  Lines        326961   326905      -56     
  Branches      47296    47280      -16     
============================================
- Hits         239684   239236     -448     
- Misses        68026    68471     +445     
+ Partials      19251    19198      -53     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[sandeshkr419]

LGTM!

[github-actions]

✅ Gradle check result for 05a24d0: SUCCESS