fix(solr): force HTTP/1.1 in SolrJ client to avoid flaky H2 EOF

[adityamparikh]

Summary

The JDK 25 HttpClient's HTTP/2 transport intermittently closes reused connections with java.io.EOFException against Solr/Jetty, causing test flakiness on CI runners. Recently observed in:

• SearchServiceIntegrationTest.testSpecialCharactersInQuery (older report)
• SearchServiceIntegrationTest.testCombinedSortingAndFiltering (today, native CI run)
• IndexingServiceIntegrationTest.testIndexJsonDocumentsWithNestedObjects (today, second attempt)

Each run picks a different victim test — the common thread is Caused by: java.io.EOFException at Http2Connection.java:1903.

HTTP/2 multiplexing is not needed for our usage (a single MCP server talking to Solr); force HTTP/1.1 on HttpJdkSolrClient via useHttp1_1(true) for deterministic behavior.

Diff

One-line change: add .useHttp1_1(true) to the HttpJdkSolrClient.Builder chain in SolrConfig.solrClient(). Plus a comment explaining why.

-		return new HttpJdkSolrClient.Builder(url).withConnectionTimeout(CONNECTION_TIMEOUT_MS, TimeUnit.MILLISECONDS)
-				.withIdleTimeout(SOCKET_TIMEOUT_MS, TimeUnit.MILLISECONDS).withResponseParser(jsonResponseParser)
-				.withRequestWriter(new XMLRequestWriter()).build();
+		return new HttpJdkSolrClient.Builder(url).withConnectionTimeout(CONNECTION_TIMEOUT_MS, TimeUnit.MILLISECONDS)
+				.withIdleTimeout(SOCKET_TIMEOUT_MS, TimeUnit.MILLISECONDS).useHttp1_1(true)
+				.withResponseParser(jsonResponseParser).withRequestWriter(new XMLRequestWriter()).build();

Background

This fix was authored locally as part of the fix/solrj-force-http1 branch back in May 2026 but never landed upstream. It's been carried in derivative branches and rediscovered today when the same flake bit a different test in CI for ongoing native-image work.

Test plan

• CI green (no transient EOFException from Http2Connection.java)
• No functional regression: SolrJ continues to talk to Solr via HTTP/1.1, which is fully supported by Solr/Jetty

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