fix: Retain exception that triggered Dart query cancellation.

[gianm]

In some cases, cancellation is triggered by an exception (rather than a non-exceptional reason, like timeout or user request). This patch retains the exception and includes it in the query report.

[FrankChen021]

Severity	Findings
P0	0
P1	0
P2	1
P3	0
Total	1

Reviewed 8 of 8 changed files.

Findings that could not be attached inline:

• multi-stage-query/src/main/java/org/apache/druid/msq/exec/ControllerHolder.java:249 - [P2] Preserve cancellation causes before the controller starts. cancel(reason, cause) only records cancelReason while the holder is still ACCEPTED. If the Dart result sequence closes or throws before the run executor starts the controller, the later runAsync path skips controller.stop(...) and builds makeCanceledReport(cancelReason), so the exception passed from DartQueryMaker is dropped and the report is still a bare UNKNOWN cancellation. Store the cause in the holder and use it when building the pre-run canceled report.

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This is an automated review by Codex GPT-5.5

[capistrant]

Franks auto review comment looks accurate, but it feels like an improvement that could be a follow if you don't want to work on implementing the idea now