fix: make main compile against the forked OpenSearch package

[marevol]

main does not compile since #3439.

Two things the migration missed

SearchEngineClient.pitSegments overrides nothing. #3439 rewrote every
org.opensearch.* reference to org.codelibs.fesen.opensearch.*. The rename was
applied uniformly, but not every type survived the fork — PitSegmentsRequest did
not, and fesen-httpclient's Client no longer declares pitSegments:

SearchEngineClient.java:[97] cannot find symbol
  symbol:   class PitSegmentsRequest
  location: package org.codelibs.fesen.opensearch.action.admin.indices.segments
SearchEngineClient.java:[3513] method does not override or implement a method from a supertype

The method and its two imports are removed. Every other
org.codelibs.fesen.opensearch.* name in the tree was checked against the 2,491
classes the fork ships; this was the only one missing.

AbstractDocumentSearcherTest was never migrated. It arrived with #3440, after
the migration branch was cut, so its six org.opensearch.* imports stayed as they
were. They are rewritten now. grep -rn 'org\.opensearch\.' src/ returns nothing.

#3439's CI could not have caught either one. It was red for its whole life because
the forked fesen-httpclient was unpublished, so fess never reached javac there.

This needs codelibs/fesen-httpclient#46 published first

That test builds a SearchResponse by hand and calls four members the fork had
pruned — SearchHits.empty(boolean), ShardSearchFailure(Exception), and one
constructor each on InternalSearchResponse and SearchResponse. #46 puts them
back. Until it is published, CI here will fail on those four symbols and nothing
else.

Verification

With #46 installed locally: mvn test-compile — BUILD SUCCESS, from a
compilation failure on main.

[marevol]

The CI failure here is not this change — it is a dependency that had not been
published yet when the run started.

CrawlerEngineClient.java:34: error: cannot access Client
public class CrawlerEngineClient extends FesenClient {
  class file for org.opensearch.transport.client.Client not found

That run started at 14:31. codelibs/fess-crawler#209 merged at 14:34 and its
snapshot was published at 14:39, so the run resolved the fess-crawler from
before the fork migration, whose FesenClient still implements the old Client.

What still has to land first

dependency	snapshot published	state
fesen-httpclient:3.8.1-SNAPSHOT	14:35	current, includes codelibs/fesen-httpclient#46
fess-crawler-opensearch:15.9.0-SNAPSHOT	14:39	current, includes codelibs/fess-crawler#209
fess-suggest:15.9.0-SNAPSHOT	yesterday 19:09	stale — codelibs/fess-suggest#98 is still open

So a re-run now gets past CrawlerEngineClient and then fails in SuggestHelper
instead: the published fess-suggest still exposes org.opensearch.transport.client.Client
in its API, and SuggestHelper passes a SearchEngineClient to it.

codelibs/fess-suggest#98 has to be merged and published before this can go green.
There is nothing further to change in this PR.

Verified locally, with all three dependencies at the right versions

• mvn test-compile — BUILD SUCCESS (a compilation failure on main).
• mvn javadoc:jar — BUILD SUCCESS. This is the goal that failed above, so it is
  worth stating separately: the gate passes once the dependency is current.