Add enums_in_transition support for gradual enum migration

[ashkan25]

Summary

When type_name_overrides is used to collapse input enum names onto output enum names (e.g., "ColorInput" => "Color"), the standard 3-stage enum value migration becomes impossible — removing the override simultaneously renames all filter field types, breaking all clients at once with no transition period.

This PR adds an enums_in_transition configuration option that enables gradual migration:

• Generates a transition *Input enum type alongside the collapsed output enum
• Adds an equalToAnyOfInput filter field that accepts the new input enum type
• Processes equalToAnyOfInput identically to equalToAnyOf at runtime (same ES terms query, same null handling, same shard routing optimization)
• Validates that enums_in_transition names match defined enum types, with DidYouMean suggestions

Migration workflow this enables

1. Add enum to enums_in_transition — schema gains ColorInput enum and equalToAnyOfInput filter field
2. Clients migrate from equalToAnyOf: [Color] to equalToAnyOfInput: [ColorInput] at their own pace
3. Remove the type_name_overrides entry — equalToAnyOf now naturally uses [ColorInput]
4. Clients migrate back to equalToAnyOf (or keep using equalToAnyOfInput as a permanent alias)
5. Remove enum from enums_in_transition

Key mechanisms

• skip_name_override on EnumType constructor — bypasses type_name_overrides for transition input enum creation
• type_already_final on Field — bypasses to_final_form for filter field type resolution
• Shared lambda in FilterNodeInterpreter for both equalToAnyOf and equalToAnyOfInput
• RoutingPicker, IndexExpressionBuilder, and Filters all handle the new operator for shard routing and index pruning
• Apollo and Warehouse factory extensions forward **kwargs so skip_name_override reaches the base method
• All RBS type signatures updated for new parameters and methods

Test plan

• All existing elasticgraph-schema_definition unit tests pass (1620 examples)
• All existing elasticgraph-graphql unit tests pass (1336 examples)
• All existing elasticgraph-schema_artifacts unit tests pass (105 examples)
• New tests verify transition input enum generation with type_name_overrides active
• New tests verify equalToAnyOfInput filter field on both standard and list element filter types
• New tests verify no transition types generated when enum is not in enums_in_transition
• Tests run with both camelCase and snake_case casing forms
• Integration tests with a running datastore (pending)

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[myronmarston]

Can you write up a proposal for this as a discussion? I'd like to the context for the problem you're trying to solve, the options you considered, why you chose this option, etc.

In general, we ask that anything with interesting design choices and tradeoffs start as a proposal.