[BUG] Fix in-process evaluation consistency: custom operator edge cases and error handling

[toddbaert]

Sub-issue of open-feature/flagd#1904. See the parent issue for full details on each bug.

Fix in-process evaluation consistency for:

• [BUG] sem_ver operator parsing behavior differs across SDK implementations flagd#1873 — sem_ver parsing/comparison
• [BUG] Custom operator error return values are inconsistent across SDK implementations flagd#1874 — custom operator error return values
• [BUG] $evaluators/$ref resolution: no parse-time validation, non-deterministic ordering, and missing Rust support flagd#1875 — $evaluators/$ref resolution

All custom operator errors must return null (or language equivalent) to ensure errors trigger a default variant fallback, not a false branch.

The flagd-testbed gherkin suite is being enhanced to cover these edge cases (open-feature/flagd-testbed#339). Once available, run the new scenarios and fix any failures.

Note: fractional bucketing differences will be addressed separately by open-feature/flagd#1903.

@toddbaert will coordinate the release across providers.