Add explain support for analytics engine path

[ahkcs]

Summary

Adds explain support for the analytics engine path (#5247). This is the second PR building on #5267 (query routing and execution handoff).

This PR covers Step 3 (explain) from the issue:

Step 3: Explain support [Done]

Return logical RelNode plan via _plugins/_ppl/_explain, include physical plan if Analytics engine provides QueryPlanExecutor.explain() API.

• AnalyticsExecutionEngine.explain(): Returns logical plan via RelOptUtil.toString() with mode-aware SqlExplainLevel (SIMPLE/STANDARD/COST). Physical and extended plans are null since the analytics engine is not yet available.
• RestUnifiedQueryAction.explain(): New entry point for explain requests on the analytics path. Formats response via JsonResponseFormatter with normalizeLf().
• TransportPPLQueryAction routing: Checks isExplainRequest() for analytics indices and routes to unifiedQueryHandler.explain() instead of execute().

Explain response format

{
  "calcite": {
    "logical": "LogicalSystemLimit(...)\n  LogicalProject(ts=[$0], message=[$2])\n    LogicalFilter(condition=[=($1, 200)])\n      LogicalTableScan(table=[[opensearch, parquet_logs]])\n",
    "physical": null,
    "extended": null
  }
}

Physical plan will be populated when the analytics engine provides a QueryPlanExecutor.explain() API.

Changed Files

File	Change
core/.../analytics/AnalyticsExecutionEngine.java	Added explain(RelNode, ExplainMode, ...) method
plugin/.../rest/RestUnifiedQueryAction.java	Added explain() entry point and createExplainListener()
plugin/.../transport/TransportPPLQueryAction.java	Route explain requests for analytics indices
core/.../analytics/AnalyticsExecutionEngineTest.java	Added 2 explain unit tests
integ-test/.../ppl/AnalyticsPPLIT.java	Added 5 explain integration tests

Test plan

• 2 unit tests -- explain logical plan, explain error propagation
• 5 integration tests -- explain response structure (JSON shape, null physical/extended), project+scan, filter, aggregation, sort plans

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