Fix route options serialization to preserve explicitly removed values

[hoffmaen]

Summary

When a route option (e.g., loadbalancing) is explicitly removed via the API, the value is set to nil internally. The options= setter previously called .compact on the options hash, which removed nil keys entirely from the serialized JSON. This made the resulting "options":{} indistinguishable from a route that never had options set, preventing downstream consumers (e.g., gorouter) from detecting the explicit removal and reverting to the platform default.

Fixes cloudfoundry/routing-release#551

Changes

• Replace .compact with .transform_values { |v| v.nil? ? '' : v } in Route#options_with_serialization=, so that explicitly removed options are serialized as empty strings (e.g., "options":{"loadbalancing":""}) instead of being dropped
• Update assertions in route_update_spec.rb and manifest_route_update_spec.rb to expect { 'loadbalancing' => '' } instead of {}
• Add a new model-level test in route_spec.rb that explicitly verifies nil option values are serialized as empty strings

Context

This is the Cloud Controller counterpart to a gorouter issue (cloudfoundry/routing-release#551) that adds support for reverting a per-route load balancing algorithm to the platform default. Without this CC change, the removal signal is lost before it reaches gorouter via NATS.

• I have reviewed the contributing guide

• I have viewed, signed, and submitted the Contributor License Agreement

• I have made this pull request to the main branch

• I have run all the unit tests using bundle exec rake

• I have run CF Acceptance Tests