Fix Exp/Ln/Log to return null on non-representable results (#1716)

[c-schuler]

Problem

Exp(1000) and Ln(0) threw exceptions instead of returning null. The CQL spec is explicit for all three operators:

"If the result of the operation cannot be represented, the result is null."

• Exp(1000) = e¹⁰⁰⁰ = ∞ -> BigDecimal(Double.POSITIVE_INFINITY) threw NumberFormatException, re-wrapped as UndefinedResult.
• Ln(0) = −∞ -> threw UndefinedResult (while Ln(-1) already returned null - internally inconsistent).
• Log(0, b) / Log(neg, b) -> uncaught NumberFormatException (no guard at all).

Fix

Replaced the exception-based handling with an explicit isInfinite() / isNaN() check that returns null when the result can't be represented as a Decimal. This also removes the UndefinedResult machinery (net-negative lines) and closes the Log gap.

• Exp(1000) -> null
• Ln(0) -> null, Ln(-1) -> null (now consistent)
• Log(0, 2) -> null

Tests

Un-doctored the vendored engine-fhir copy of CqlArithmeticFunctionsTest.xml - these had been marked invalid="true" to match the old throwing behavior:

Test	Expression	Before	After
Exp1000	Exp(1000)	invalid	null
Exp1000D	Exp(1000.0)	invalid	null
Ln0	Ln(0)	invalid	null
LnNeg0	Ln(-0)	invalid	null

Validated: spotlessApply, :engine:detekt, full :engine:jvmTest, full :engine-fhir:test - all green.

Upstream follow-up

The canonical cql-tests (https://github.com/cqframework/cql-tests) suite still marks Exp1000 and Ln0 as invalid="true", which now contradicts the explicit spec clause above. A companion PR to cql-tests is needed to change these to expect null; until then the conformance dashboard will report them as failing.

Fixes #1716.

[github-actions]

Related Issues

The following open issues may be related to this PR:

Issue	Title	Score	Matched Terms
#1733	Operation Predecessor DateTime and Time underflow does not return null	40.5	"result null", "cql tests", "represented result", "result operation", "cannot represented", "operation cannot", "return null", "invalid true", result, xml, true, return, expect, represented, invalid, operation, null, expression, https, cannot, tests, representable, fhirpath (path), org (path), cql (path), hl7 (path), test (path)
#1734	Operation Predecessor DateTime and Time overflow does not return null	39	"result null", "cql tests", "represented result", "result operation", "cannot represented", "operation cannot", "return null", "invalid true", result, true, return, represented, above, invalid, operation, null, expression, https, cannot, tests, representable, org (path), cql (path), hl7 (path), test (path)
#1400	patientBirthDatePropertyName marked as deprecated but still required for SystemFunctionResolver	33.5	"com cqframework", "cql spec", "https github", "github com", "cqframework cql", non, clause, instead, cqframework, github, com, marked, expect, problem, behavior, spec, null, https, returned, cannot, src (path), functions (path), main (path), org (path), cql (path), elm (path), hl7 (path)
#1377	Use a git module for the XML-based tests	31.5	"com cqframework", "cql tests", "fhir test", "https github", "github com", "engine fhir", "cqframework cql", xml, cqframework, github, com, fhir, https, suite, based, tests, engine (path), cql (path), test (path)
#1794	Point from Interval[null, null] is returning null instead of erroring	30.5	"null result", "instead returning", "invalid true", "null null", "returning null", result, xml, true, instead, return, returning, operators, invalid, spec, null, expression, https, tests, fhirpath (path), org (path), cql (path), hl7 (path), test (path)

Tip: If this PR addresses any of these issues, please link them using Closes #NNN or Refs #NNN in the PR description.

[github-actions]

Formatting check succeeded!

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.76%. Comparing base (95366e9) to head (80e0b41).

Files with missing lines	Patch %	Lines
...encds/cqf/cql/engine/elm/executing/ExpEvaluator.kt	0.00%	4 Missing ⚠️
...pencds/cqf/cql/engine/elm/executing/LnEvaluator.kt	50.00%	1 Missing and 1 partial ⚠️
...encds/cqf/cql/engine/elm/executing/LogEvaluator.kt	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1804   +/-   ##
=========================================
  Coverage     68.75%   68.76%           
  Complexity     1699     1699           
=========================================
  Files           536      536           
  Lines         30559    30555    -4     
  Branches       7013     7012    -1     
=========================================
- Hits          21011    21010    -1     
+ Misses         6994     6989    -5     
- Partials       2554     2556    +2     

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
43.5% Coverage on New Code
0.0% Duplication on New Code

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