RPC peers: emit empty arrays for descriptor collections, never omit them

[jkschneider]

What

Recipe.getDescriptor() on the Java side always returns a descriptor whose collection-valued getters (tags, options, preconditions, recipeList, dataTables, maintainers, contributors, examples) are non-null. Callers across the ecosystem rely on that and iterate the getters directly:

for (RecipeDescriptor precondition : descriptor.getPreconditions()) { ... }
descriptor.getPreconditions().stream().map(...).toList();

When a RecipeDescriptor arrives from a polyglot RPC peer, the JSON that peer emits can omit empty collections — Jackson on the Java side then leaves the corresponding fields null, and downstream code blows up:

Cannot invoke "java.util.List.iterator()" because the return value of
"org.openrewrite.config.RecipeDescriptor.getPreconditions()" is null

How

Fix at the source — each polyglot peer always emits every collection key, even when empty:

Python (rewrite-python/rewrite/src/rewrite/rpc/server.py)
_recipe_descriptor_to_dict now emits preconditions, maintainers, contributors, examples as empty lists. The Python RecipeDescriptor dataclass doesn't model these fields yet, so they're hardcoded [].

TypeScript (rewrite-javascript/rewrite/src/recipe.ts)
Extended the RecipeDescriptor interface with the four missing fields and updated Recipe.descriptor() to populate them. Updates the existing recipe.test.ts snapshot.

C# — no change. RecipeDescriptorDto already declares each list/set property with an = [] initializer and Newtonsoft serializes empty collections as [], so the JSON it produces already matches the contract.

Test plan

• rewrite-python/rewrite/tests/rpc/test_server.py::test_recipe_descriptor_to_dict_emits_all_collection_keys — locks the contract on the Python side.
• rewrite-javascript/rewrite/test/recipe.test.ts — updated snapshot now asserts the four empty-collection keys.
• :rewrite-javascript:compileJava :rewrite-csharp:compileJava :rewrite-python:compileJava pass.