Improve error message for unsupported window functions in eventstats/streamstats

[gingeekrishna]

Description

Fixes #5168

eventstats/streamstats reject window functions outside WINDOW_FUNC_MAPPING (e.g. rank(), dense_rank(), nth_value()) with a bare "Unexpected window function: X" error.

As confirmed in the issue discussion by @songkant-aws, this is expected behavior, not a bug — rank/dense_rank/nth_value require ORDER BY semantics, but eventstats/streamstats only support partition by. The issue was relabeled error-experience: the fix is to make the error message clearer, not to add support for these functions.

Changes

• CalciteRexNodeVisitor#visitWindowFunction: replaced the generic "Unexpected window function: X" message with one that names the rejected function and lists the functions eventstats/streamstats do support (sourced from WINDOW_FUNC_MAPPING), so users get actionable guidance instead of a bare error.
• Updated the existing unit test (UnifiedQueryPlannerTest) and integration tests (CalcitePPLEventstatsIT, CalciteStreamstatsCommandIT) to assert against the new message.
• Added new integration tests covering rank/dense_rank specifically, since the issue's repro queries used those functions.

Note: this PR builds on top of #5587 (already merged), which fixed the same throw site to return a 4xx instead of a 500. This PR only changes the message text, not the exception type or HTTP status.

Test plan

• Unit test UnifiedQueryPlannerTest#unsupportedWindowFunctionIsRethrownAsSemanticCheckException passes
• Integration tests added/updated for eventstats and streamstats (require a live cluster to run in CI)

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🧪 PR contains tests

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⚡ Recommended focus areas for review Hardcoded list The error message hardcodes the list of supported window functions as a string literal. If WINDOW_FUNC_MAPPING changes (functions added or removed), this message will become stale and misleading. The list should be derived programmatically from WINDOW_FUNC_MAPPING.keySet() to stay accurate. () -> new CalciteUnsupportedException( "Window function '" + funcName + "' is not supported in eventstats/streamstats." + " Supported functions: avg, count, dc, distinct_count, earliest," + " latest, max, min, row_number, stddev_pop, stddev_samp, sum," + " var_pop, var_samp."));

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PR Code Suggestions ✨

Explore these optional code suggestions:

Category	Suggestion	Impact
General	Dynamically generate supported functions list The hardcoded list of supported functions in the error message may become outdated if new functions are added to WINDOW_FUNC_MAPPING. Consider dynamically generating this list from the mapping keys to ensure accuracy and maintainability. core/src/main/java/org/opensearch/sql/calcite/CalciteRexNodeVisitor.java [709-715] new CalciteUnsupportedException( "Window function '" + funcName + "' is not supported in eventstats/streamstats." - + " Supported functions: avg, count, dc, distinct_count, earliest," - + " latest, max, min, row_number, stddev_pop, stddev_samp, sum," - + " var_pop, var_samp.")); + + " Supported functions: " + + String.join(", ", WINDOW_FUNC_MAPPING.keySet()) + + ".")); Suggestion importance[1-10]: 7 __ Why: This suggestion improves maintainability by dynamically generating the list of supported functions from WINDOW_FUNC_MAPPING.keySet() instead of hardcoding them. This ensures the error message stays accurate when new functions are added, reducing maintenance burden and preventing outdated documentation.	Medium