[Backport 2.x] Reject invalid SearchBackpressureMode (#6832) (#7541)

[andrross]

• Reject invalid SearchBackpressureMode ([BUG] Incorrect SearchBackpressureMode can corrupt cluster state #6832)

ClusterState updates of SearchBackpressureMode are persisted without validation which causes the entire cluster to become unstable or corrupted when a cluster tries to load invalid mode.

Signed-off-by: Austin Lee [email protected]
(cherry picked from commit ba35781)

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/15436/
• CommitID: 1827c5e
  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 (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/15437/
• CommitID: 4573021
  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 (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/15453/
• CommitID: 7456c86

[codecov]

Codecov Report

Merging #7571 (7456c86) into 2.x (ead9495) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                2.x    #7571      +/-   ##
============================================
- Coverage     70.38%   70.31%   -0.07%     
+ Complexity    59990    59933      -57     
============================================
  Files          4882     4882              
  Lines        288376   288376              
  Branches      41920    41920              
============================================
- Hits         202961   202778     -183     
- Misses        68430    68606     +176     
- Partials      16985    16992       +7     

Impacted Files	Coverage Δ
.../backpressure/settings/SearchBackpressureMode.java	100.00% <100.00%> (+15.38%)	⬆️
...kpressure/settings/SearchBackpressureSettings.java	92.59% <100.00%> (ø)

... and 484 files with indirect coverage changes