test: define malformed condition parse errors

[leoromanovsky]

Motivation

Malformed configured condition operands do not have complete canonical coverage, which allows downstream SDKs to disagree between DEFAULT and ERROR outcomes. This was surfaced while reviewing DataDog/dd-trace-py#19390.

Changes

• Add isolated fixtures for GT with a string operand, ONE_OF with a scalar operand, and IS_NULL with a non-boolean operand.
• Add catch-all traps so implementations cannot silently skip an invalid condition and continue evaluating the flag.
• Keep a valid neighboring flag to verify that a malformed flag does not poison the full configuration.
• Make PARSE_ERROR explicit for malformed condition operands and the existing invalid configured SemVer comparand.
• Align the invalid-regex fixture with the same ERROR/PARSE_ERROR contract.
• Document the optional canonical result.errorCode field.

Decisions

• Treat malformed configured condition operands as per-flag parse errors.
• Return the caller default with reason: ERROR and errorCode: PARSE_ERROR for the affected flag.
• Preserve per-flag isolation so valid flags in the same configuration continue evaluating normally.
• Intentionally change the recently added invalid-regex expectation from DEFAULT to ERROR so it matches system-test expectations and the other configured-condition parse failures.

Validation

• python3 ci/validate-fixtures.py
• git diff --check

[leoromanovsky]

Superseded by #26, which now contains the combined malformed-flag isolation fixtures.