[CALCITE-7731] Bound plain-notation expansion of DECIMAL literals to prevent parse-time OutOfMemoryError (follow-up, added more tests)

[skrcode]

Follow-up to #5204, requested by @rubenada in #5204 (comment).

This adds direct regression tests for the three CALCITE-7731 plain-notation guards outside the parser path:

• RexBuilder.makeLiteral with a dialect-permitted negative DECIMAL scale
• Rel-to-SQL conversion of an oversized exact DECIMAL literal
• SqlNumericLiteral.toValue for a directly constructed literal

Each test uses a bounded input of roughly 20,000 digits and verifies the specific guard exception. No production code changes are included.

Validation on current main with Java 21 and the pinned Gradle 8.14.4 wrapper:

• focused new tests: 3 passed, 0 failed
• RelToSqlConverterTest: 623 passed, 0 failed
• RexBuilderTest: 72 passed, 0 failed
• SqlNodeTest: 4 passed, 0 failed
• core Checkstyle and AutoStyle: passed
• :core:build: 16,658 completed, 0 failed, 155 skipped
• repository-wide build: BUILD SUCCESSFUL, 413 tasks

The initial missing cases were identified by JAIPilot Cloud, then rebased, reviewed, and independently validated against the latest upstream main.

[rubenada]

@skrcode thanks for the patch! LGTM
Could you please squash commits into a single one? As final commit message I'd propose something like: [CALCITE-7731] Bound plain-notation expansion of DECIMAL literals to prevent parse-time OutOfMemoryError (follow-up, added more tests)

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[rubenada]

Thanks for the patch @skrcode