minor(cleanup): fix 'occurence'/'seperate' typos in comments and javadocs

[singhvishalkr]

Fixes #18226. This is a targeted, behavior-neutral sweep of the low-risk typos called out in the issue scope ("Fix obvious typos and grammar issues in code comments and Javadocs. Keep the sweep low-risk and behavior-neutral.").

Scope of this PR

• occurence -> occurrence in javadoc and line comments only. Left all parameter names (int occurence, @param occurence) untouched because those are part of the scalar function's public surface and renaming them would be a user-visible change.
• seperate -> separate in config-file comments only. Left the test method names testStartSeperately / testStartSeperatelyWithStreamingUntar untouched so build filters and historical test-report links aren't invalidated.

Files changed

• pinot-common/src/main/java/org/apache/pinot/common/function/scalar/StringFunctions.java -- Javadoc for four strpos-style overloads ("Return the Nth occurrence of a substring...").
• pinot-common/src/main/java/org/apache/pinot/common/function/scalar/regexp/RegexpReplaceConstFunctions.java -- Javadoc "all occurrences. Match is performed in case-sensitive mode."
• pinot-integration-tests/src/test/java/org/apache/pinot/integration/tests/OfflineClusterIntegrationTest.java -- two // Test occurrence comments.
• pinot-integration-tests/src/test/java/org/apache/pinot/integration/tests/MultiStageEngineIntegrationTest.java -- two // Test occurrence comments.
• pinot-core/src/test/resources/conf/pinot-broker.properties -- # Comma separated list of servers....
• pinot-server/src/test/resources/conf/pinot-broker.properties -- same comment.

Deliberately out of scope

• The supproted typo originally quoted from RefreshSegmentTaskExecutor.java is no longer in that file on master (already fixed at some point since the issue was filed), so nothing to do there.
• Stale TODO / FIXME wording clean-ups require per-site engineering judgement and were left for a follow-up sweep.
• successfull still has around 30 hits but many are embedded in log keys, downstream-parsed status strings, and compressor enum names where a rename isn't comment-only; skipping for safety.
• Parameter names (occurence in public SQL-reachable function signatures) are deliberately untouched to stay behavior-neutral.

Verification

• No code paths changed; only comment and property-file comment text.
• Labels from the PR template that apply: cleanup, code-style, testing (two test files touched for comment text only).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.61%. Comparing base (73c89c7) to head (63f3664).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #18302      +/-   ##
============================================
+ Coverage     63.59%   63.61%   +0.01%     
  Complexity     1659     1659              
============================================
  Files          3247     3247              
  Lines        197495   197495              
  Branches      30569    30569              
============================================
+ Hits         125606   125644      +38     
+ Misses        61840    61803      -37     
+ Partials      10049    10048       -1     

Flag	Coverage Δ
custom-integration1	100.00% <ø> (ø)
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (ø)
java-11	63.59% <ø> (+0.02%)	⬆️
java-21	63.56% <ø> (-0.01%)	⬇️
temurin	63.61% <ø> (+0.01%)	⬆️
unittests	63.61% <ø> (+0.01%)	⬆️
unittests1	55.57% <ø> (+0.02%)	⬆️
unittests2	35.07% <ø> (+<0.01%)	⬆️

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