fix: various custom operator conformance fixes

[toddbaert]

fix: various custom operator conformance fixes

• support v/V prefix in semver
• support partial versions in semver
• support numeric context values in semver
• return null on errors
• fix fractional single-entry flattening

⚠️ wait until flagd/RPC is updated to merge.

Fixes: #1774

[gemini-code-assist]

Code Review

This pull request introduces several improvements to the flagd core evaluation logic, including enhanced type handling for context values, more robust SemVer parsing with support for partial versions and numeric coercion, and fixes for fractional evaluation edge cases. I have provided a suggestion to optimize the fractional evaluation logic by using List operations instead of manual array copying to improve performance.

[gemini-code-assist]

The current implementation performs multiple array copies and manual list building, which can be inefficient as this code is executed during every flag evaluation. Using List.subList and List.addAll is more efficient and idiomatic for this purpose.

            List<Object> remaining = arguments.subList(1, arguments.size());

            // json-logic pre-evaluation flattens a single-entry fractional
            // e.g. [["single",1]] becomes ["single", 1]; detect and re-wrap
            if (!remaining.isEmpty() && !(remaining.get(0) instanceof List)) {
                List<Object> wrapped = new ArrayList<>(remaining.size() + 1);
                wrapped.add(arg1);
                wrapped.addAll(remaining);
                distributions = Arrays.asList((Object) wrapped);
            } else {
                distributions = remaining;
            }