Deduplicate generated schema artifacts using pydantic:module annotation

[jmchilton]

Context

The schema_dedup branch moved shared types (creators, ToolShedRepository, StepPosition, HasUUID, etc.) into schema/common/common.yml — but the codegen still expands them inline into every generated module. This means gxformat2.py and native.py each contain their own copy of every common type, and _conversion.py has duplicate conversion functions to marshal between structurally-identical-but-different classes.

Proposal

Add a pydantic:module annotation to schema-salad-plus-pydantic so types can declare which Python module they belong to. When generating a module, types annotated with a different pydantic:module emit an import instead of a class definition.

Schema side

# common.yml
- name: StepPosition
  type: record
  pydantic:module: "gxformat2.schema.common"
  fields: ...

CLI side

New --module-name arg so the generator knows which module it's building:

# Generate common module
schema-salad-plus-pydantic generate schema/common/common.yml \
    --module-name gxformat2.schema.common -o gxformat2/schema/common.py

# Format-specific modules import common types instead of inlining
schema-salad-plus-pydantic generate schema/v19_09/workflow.yml \
    --module-name gxformat2.schema.gxformat2 -o gxformat2/schema/gxformat2.py

Application side

• Import common types from gxformat2.schema.common instead of aliased duplicates from both modules
• Collapse duplicate conversion functions (_convert_creators / _convert_creators_to_native, _convert_tool_shed_repo_to_format2 / _convert_tool_shed_repo_to_native)

Open questions

• Abstract types in common module — emit as abstract BaseModel or skip?
• Strict variants — separate common_strict.py or share one common module?
• load_document() / _load_single() require document roots — common module should skip (library mode)?
• pydantic: namespace declaration in common.yml (currently only declared in importing workflow.yml files)
• Traditional codegen (v19_09.py, native_v0_1.py) — leave as-is or drop? Nothing imports them.

[jmchilton]

This is regards to the work started here: #165. I'm not sure how I feel about how clean we can make this separation happen - the plan above I'm not in love with and this feels like a low priority issue.