refactor: Convert nullable Embind types to use std::optional

[coderabbitai]

Background

Currently, we register nullable types in Embind using custom type definitions that map to 'T | null' in TypeScript. For example:

• FilteredLogEventMapTsType -> number[] | null
• LogEventIdxTsType -> number | null

Embind now provides built-in support for std::optional<T> through the register_optional() function, as documented here.

Proposed Changes

Convert all nullable type registrations to use std::optional<T> with register_optional().

References

• Discussion: /feat: Support search logs by timestamp for structured and unstructured logs. #42#discussion_r1905740583