feat: use tl::unexpected in Index default implementations

Replace throw std::runtime_error with LOG_ERROR_AND_RETURNS in virtual
method default implementations that return tl::expected<T, Error>.

This provides consistent error handling - callers can use .has_value()
and .error() instead of catching exceptions.

Signed-off-by: LHT129 <lht@antgroup.com>
Assisted-by: OpenCode:mini-max-M2.7

Author: LHT129