Select the service and shapes to generate

[jonathan343]

Note

This PR is stacked on #795 and targets python-codegen-model so it contains only service and shape selection. I will retarget to develop after #795 merges.

Summary

This PR adds --service to both native generator commands. An explicit ID must identify a non-mixin service; otherwise the sole non-mixin service is selected automatically. Client generation requires a service, while standalone types can operate without one.

Selection follows the service's structural relationships, including resources, operation inputs/outputs/errors, and member targets. It preserves model order, handles cyclic and deep graphs, and excludes prelude shapes, mixins, trait definitions, and service/operation/resource shapes. Without a service, conflicting case-insensitive data-shape names are reported. The CLI reports how many eligible shapes lie outside the selected service closure.

The selection API and behavior are documented in designs/codegen/selection.md.

Usage

# Automatically select the model's sole service
smithy-python generate client --model weather.json --output ./build

# Select a service explicitly when the model contains several
smithy-python generate client --model services.json \
  --service example.weather#Weather --output ./build

# Select standalone data shapes from a model without a service
smithy-python generate types --model types.json --output ./build

Both commands limit selection to the resolved service's reachable data shapes. When no service exists, types selects all eligible data shapes and rejects case-insensitive name conflicts.

When the model contains multiple non-mixin services, omitting --service fails for both commands and lists the candidates. For example:

smithy-python generate types --model services.json --output ./build

smithy-python: error: Multiple services found; select one with --service: example.weather#Weather, example.inventory#Inventory

The command exits with code 2. Pass --service example.weather#Weather to select that service's reachable data shapes.

Additional Testing

All 434 models in aws/api-models-aws passed an independent raw-AST traversal comparison: 119,385 selected shapes, exact ordering and exclusion counts, and matching explicit/automatic service selection. Both CLI artifacts were exercised for every service, totaling 868 invocations. This is local validation, not an external CI dependency.

These models contain no mixins, apply, or service renames, and each has only one service; those selection scenarios are covered by targeted tests. Additional local probes checked structural relationships, ordering, cycles, and deep graphs.

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