fix: increase AVG MathContext precision from 10 to 34 to prevent BIGINT overflow and silent precision loss

[waterWang]

Problem

#39751 — SQL AVG rounds every result to 10 significant digits via the fixed MathContext(10, RoundingMode.HALF_UP) at BeamBuiltinAggregations.java:93. All AVG subtypes (INT32, INT64, INT16, INT8, FLOAT, DOUBLE, DECIMAL) go through prepareOutput which divides the accumulator with this precision.

Consequences:

• AVG over a single BIGINT value 9223372036854775807 returns -9223372036709551616 — sign flipped due to 10-digit rounding then .longValue() truncation
• Values above 10 significant digits (ids, epoch-millis, cent-denominated amounts) are silently rounded (e.g. AVG(1786500000123) → 1786500000000)
• DECIMAL(18,2) values lose precision (123456789.99 → 123456790.0)

Fix

Increase the MathContext precision from 10 to 34 significant digits (MathContext.DECIMAL128 precision, with the same HALF_UP rounding mode). This keeps all values representable in any of the supported input types exact through the division:

• INT64/BIGINT values need up to 19 significant digits — 34 is sufficient, so the sign flip and truncation disappear
• DECIMAL(38,10) can need up to 49 digits, but 34 covers the overwhelmingly common DECIMAL(p,s) ranges and matches MathContext.DECIMAL128 semantics (sum/count where the division terminates is exact; non-terminating divisions are rounded to 34 digits instead of 10)

The alternative of dropping the MathContext would turn non-terminating divisions into ArithmeticException; threading the declared output RelDataType precision/scale from AggregateCall is a larger design change touching VAR_*, STDDEV_*, COVAR_* coders — a follow-up for the component owners. This fix addresses the reported correctness bugs with a minimal, targeted change.

Fixes #39751

[github-actions]

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R: @Abacn for label java.

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