Clarify cluster node request classification

[cwperks]

What changed

• rename HeaderHelper.isInterClusterRequest() to isLocalClusterNodeRequest()
• rename HeaderHelper.isTrustedClusterRequest() to isRemoteClusterNodeRequest()
• update authorization call sites, local variables, comments, and trace messages
• add focused tests for the local and remote classifications

Why

The existing names invert current OpenSearch terminology: the
interClusterRequest transient represents a request from another node in the
local cluster, while trustedClusterRequest represents a node request from a
different cluster. The new names make security-sensitive branches easier to
audit.

This is a terminology-only change. The existing transient keys and request
classification behavior remain unchanged.

Validation

• ./gradlew :precommit -Pcrypto.standard=FIPS-140-3
• ./gradlew test --tests 'org.opensearch.security.support.HeaderHelperTests' -Pcrypto.standard=FIPS-140-3
• ./gradlew citest --tests 'org.opensearch.security.support.HeaderHelperTests' -Pcrypto.standard=FIPS-140-3

[github-actions]

PR Reviewer Guide 🔍

(Review updated until commit 67e70a8)

Here are some key observations to aid the review process:

🧪 PR contains tests

🔒 No security concerns identified

✅ No TODO sections

🔀 No multiple PR themes

⚡ No major issues detected

[github-actions]

Persistent review updated to latest commit 67e70a8

[codecov]

Codecov Report

❌ Patch coverage is 54.16667% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.41%. Comparing base (30e36a1) to head (67e70a8).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...org/opensearch/security/filter/SecurityFilter.java	62.50%	0 Missing and 3 partials ⚠️
.../opensearch/security/OpenSearchSecurityPlugin.java	50.00%	1 Missing and 1 partial ⚠️
.../security/action/whoami/TransportWhoAmIAction.java	0.00%	2 Missing ⚠️
...rch/security/transport/SecurityRequestHandler.java	75.00%	2 Missing ⚠️
...ecurity/privileges/dlsfls/DlsFlsLegacyHeaders.java	0.00%	0 Missing and 1 partial ⚠️
.../org/opensearch/security/support/HeaderHelper.java	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6326      +/-   ##
==========================================
+ Coverage   75.32%   75.41%   +0.09%     
==========================================
  Files         456      456              
  Lines       29900    29924      +24     
  Branches     4520     4530      +10     
==========================================
+ Hits        22522    22568      +46     
+ Misses       5289     5262      -27     
- Partials     2089     2094       +5     

Files with missing lines	Coverage Δ
...ecurity/privileges/dlsfls/DlsFlsLegacyHeaders.java	82.85% <0.00%> (ø)
.../org/opensearch/security/support/HeaderHelper.java	68.42% <0.00%> (ø)
.../opensearch/security/OpenSearchSecurityPlugin.java	84.37% <50.00%> (ø)
.../security/action/whoami/TransportWhoAmIAction.java	28.57% <0.00%> (ø)
...rch/security/transport/SecurityRequestHandler.java	57.92% <75.00%> (ø)
...org/opensearch/security/filter/SecurityFilter.java	68.60% <62.50%> (+0.12%)	⬆️

... and 11 files with indirect coverage changes

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