[Backport 2.19] Add Eclipse P2 mirror to avoid download.eclipse.org outages (security)

[peterzhuamazon]

Description

Manual backport of #6391 to the 2.19 branch. Routes the Spotless Eclipse JDT formatter provisioning through the OpenSearch CI mirror (https://ci.opensearch.org/) instead of download.eclipse.org to avoid upstream outages/timeouts. Only the Eclipse P2 mirror one-liner is backported; any unrelated changes bundled in the original PR are not included.

Issues Resolved

Backport of #6391
opensearch-project/opensearch-build#6421

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[github-actions]

PR Code Analyzer ❗

AI-powered 'Code-Diff-Analyzer' found issues on commit 66a7c74.

⛔ Hard block: Issues at High severity or above will block this PR from merging.

Path	Line	Severity	Description
gradle/formatting.gradle	17	high	Build plugin package source URL redirected: Eclipse P2 repository downloads are redirected from the official 'https://download.eclipse.org/' to 'https://ci.opensearch.org/' via withP2Mirrors(). This is a build plugin/package source URL modification that must be flagged per mandatory rules. A compromised or attacker-controlled mirror at ci.opensearch.org could serve malicious Eclipse formatter plugins or other P2 artifacts, poisoning the build pipeline for all developers and CI runs.

The table above displays the top 10 most important findings.

Total: 1 | Critical: 0 | High: 1 | Medium: 0 | Low: 0

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