Return 4xx instead of 500 for unsupported window functions

[OVI3D0]

Description

Window functions outside WINDOW_FUNC_MAPPING (e.g. RANK) were escaping the analytics-engine route as HTTP 500. The throw site in CalciteRexNodeVisitor#visitWindowFunction emitted a raw UnsupportedOperationException, which UnifiedQueryPlanner.plan rethrows unchanged, so it never got reclassified as a client error.

Switching the throw to CalciteUnsupportedException lets the 4xx wrapper from #5569 take over and normalize it to SemanticCheckException.

Repro:

SELECT RegionID, COUNT(*) AS cnt,
       RANK() OVER (ORDER BY COUNT(*) DESC) AS rnk
FROM clickbench GROUP BY RegionID LIMIT 5

Test plan

• New UnifiedQueryPlannerTest#unsupportedWindowFunctionIsRethrownAsSemanticCheckException: fails on main, passes with the fix
• Existing CalcitePPLEventstatsIT#testUnsupportedWindowFunctions still green
• Manual repro on a local cluster:

mikeovi@80a997164b98 ~ % curl -sS -w "\nHTTP %{http_code}\n" -X POST "http://localhost:9200/_plugins/_ppl" \
    -H 'Content-Type: application/json' \
    -d '{"query":"source=demo | eventstats rank() | head 5"}'
{
  "error": {
    "context": {
      "stage": "analyzing",
      "stage_description": "Parsing and validating the query"
    },
    "reason": "Unexpected window function: rank",
    "details": "Unexpected window function: rank",
    "location": ["while preparing and validating the query plan"],
    "code": "UNKNOWN",
    "type": "CalciteUnsupportedException"
  },
  "status": 400
}
HTTP 400
mikeovi@80a997164b98 ~ %

Check List

• New functionality includes testing.
• New functionality has been documented.
• Commits are signed per the DCO using --signoff.

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[github-actions]

PR Reviewer Guide 🔍

(Review updated until commit bc9bc97)

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🧪 PR contains tests

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[github-actions]

Persistent review updated to latest commit 5068696

[github-actions]

Persistent review updated to latest commit bc9bc97

[dai-chen]

Thanks for the fix!