Harden migrate tests for single-provider (no typeField) resources

[DarshitChanpura]

Description

Adds regression coverage for the resource-sharing migrate path used by plugins that register a single ResourceProvider with no typeField (for example, notifications' notification_config). This behavior currently has no unit coverage; the tests lock it so a future change to classifyDocType or the request-path validation can't silently break migration for these plugins.

Two cases:

• classifyNotificationConfigFallsBackToSingleAccessLevelKey — with no type paths, classifyDocType falls back to the sole default_access_level key, so a notification_config doc is classified (and migrated) rather than recorded under skippedResources. Uses a faithful .opensearch-notifications-config _source (NotificationConfigDoc.toXContent shape with the real NotificationConstants tag names).
• nonResolvingUsernamePathYieldsNoOwnerSoMigrationUsesDefaultOwner — username_path must be a valid, non-empty path (an empty "" is rejected by PATH_VALIDATOR#requireNonEmpty), and a valid non-resolving path yields no owner, so migration attributes ownership to default_owner while backend_roles_path (/metadata/access) still supplies the shared backend roles.

Test-only; no production change. The live write-path counterpart is already covered by MigrateApiTests (a resource with no owner migrates into resourcesWithDefaultOwner).

Testing

./gradlew test --tests "org.opensearch.security.resources.api.migrate.MigrateResourceSharingInfoApiActionTests" — 10 passed, 0 failed.

Check List

• New functionality includes testing.
• Commits are signed per the DCO using --signoff.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[github-actions]

PR Reviewer Guide 🔍

(Review updated until commit d35bb2f)

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 d35bb2f

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.75%. Comparing base (257a3c8) to head (d35bb2f).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6454      +/-   ##
==========================================
+ Coverage   75.74%   75.75%   +0.01%     
==========================================
  Files         457      457              
  Lines       30512    30508       -4     
  Branches     4615     4615              
==========================================
+ Hits        23110    23111       +1     
+ Misses       5284     5277       -7     
- Partials     2118     2120       +2     

see 8 files with indirect coverage changes

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