Add experimental Python codegen CLI scaffold

[jonathan343]

Overview

This PR lays the foundation for a Python-native Smithy code generator. It adds an experimental smithy-python package with the command-line interface that will eventually generate Python clients and standalone types packages.

The CLI currently validates its inputs and then exits with an explicit “generation is not implemented yet” error. The existing Java generator remains the primary one while the Python implementation is developed incrementally.

Background

The Python generator will consume Smithy JSON AST models and produce packages compatible with the existing Smithy Python runtime. It is distributed separately from those runtime packages and is only needed during code generation.

Smithy’s run plugin invokes the generator as an external process, allowing it to integrate with standard Smithy builds without being loaded into the Smithy CLI or implemented in Java. Read Smithy run plugin docs for more info.

Changes

• Add the smithy-python workspace package and executable.
• Add generate client and generate types commands.
• Support direct invocation using --model and --output.
  • Note: This list of params will grow as in subsequent PRs
• Support Smithy run plugin invocation through standard input and the provided plugin environment.
• Validate invocation inputs and distinguish argument errors from I/O and generation failures.
• Add unit tests covering CLI behavior, environment parsing, and the error hierarchy.
• Document the generator architecture, CLI contract, and migration strategy.

CLI smoke tests

$ smithy-python --version
smithy-python 0.0.0

$ smithy-python generate --help
usage: smithy-python generate [-h] {client,types} ...

positional arguments:
  {client,types}
    client        Generate a client package
    types         Generate a standalone types package

options:
  -h, --help      show this help message and exit

Both generation commands accept a model and reach the expected placeholder behavior:

$ echo '{}' | smithy-python generate client --output /tmp/smithy-python-client
smithy-python: error: client generation is not implemented yet

$ echo '{}' | smithy-python generate types --output /tmp/smithy-python-types
smithy-python: error: types generation is not implemented yet

Both generation commands exit with status 1, as expected for the current implementation.

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