bug(py/ai): Prompt input.default from dotprompt not passed to DevUI

[yesudeep]

Summary

The Python SDK's prompt.py drops the input.default field when constructing the prompt metadata for the reflection API. This is the same issue as #4534 but for the Python SDK.

Root Cause

In py/packages/genkit/src/genkit/blocks/prompt.py, the load_prompt_metadata() function builds the input dict without the default key:

'input': {
    'jsonSchema': input_schema.get('schema') if input_schema and isinstance(input_schema, dict) else None,
},

But dotprompt's PromptInputConfig has a default field containing default values for input variables. This field is part of the dotprompt spec (spec/variables.yaml) and is supported by all 6 implementations (JS, Python, Go, Dart, Java, Rust).

Fix

Add default to the input dict:

'input': {
    'default': input_schema.get('default') if input_schema and isinstance(input_schema, dict) else None,
    'jsonSchema': input_schema.get('schema') if input_schema and isinstance(input_schema, dict) else None,
},

Impact

• DevUI prompt input forms don't show default values from .prompt files
• Users must manually enter values that are already specified as defaults in the prompt definition

References

• JS counterpart: bug(js/ai): Prompt input.default from dotprompt not passed to DevUI #4534
• Dotprompt spec: spec/variables.yaml in google/dotprompt
• DevUI schema expects input.default: genkit-tools/common/src/types/prompt.ts:38