fix(controller): health assessment fixes

[krancour]

Fixes #4673 among other problems.

Per #4673, the post-op "cool down" period before App health status is deemed reliable was not being enforced in the (very common) case where desired revisions are not specified by the health check (because they were not specified by the argocd-update step that created the health check).

Apart from fixing that, this PR also fixes some other problems I've noted while working on this section of code:

1. We probably shouldn't count App health status as reliable if the current/last operation is in-progress or has any status other than Succeeded.

2. Pausing for the cool down period to elapse before deeming App health reliable is inadequate if we don't then repeat all checks we'd previously cleared to determine what new health problems may have arisen during the cool down period. New error conditions could have been introduced. An entirely new operation may have started.

The changes in this PR amount to the following:

1. If the current/last operation is in any state other than Succeeded, App health is immaterial and Stage health is automatically Unknown.

2. If last operation Succeeded, but fewer than 10 seconds ago, App health is immaterial and Stage health is automatically Unknown.

3. No other aspect of App health is examined until/unless the last operation Succeeded more than 10 seconds prior.

4. "Cool down," no longer occurs synchronously within the health check logic.

5. The (regular) Stage reconciler now treats Unknown Stage heath as an error condition, prompting the Stage to be requeued for reconciliation while observing a progressive backoff. This allows "cool down" to happen naturally and without blocking other pending Stage reconciliations.

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[codecov]

Codecov Report

❌ Patch coverage is 84.21053% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.38%. Comparing base (045305d) to head (2c96746).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/controller/stages/regular_stages.go	25.00%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4674      +/-   ##
==========================================
+ Coverage   53.35%   53.38%   +0.03%     
==========================================
  Files         388      388              
  Lines       32711    32718       +7     
==========================================
+ Hits        17454    17468      +14     
+ Misses      14380    14373       -7     
  Partials      877      877              

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[akuitybot]

Backport failed for release-1.6, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin release-1.6
git worktree add -d .worktree/backport-4674-to-release-1.6 origin/release-1.6
cd .worktree/backport-4674-to-release-1.6
git switch --create backport-4674-to-release-1.6
git cherry-pick -x bc6de1652fda0a30aacc505dbcc8781bb952531e