chore(py): Update package READMEs on PyPI

[huangjeff5]

Various improvements to plugin readme docs, such as:

• Standardizing on uv, correct package imports, and *-latest pattern for gemini usage
• Removing outdated Changelog reference
• Add a hero snippet if didn't exist before
• Use inline tools over strings

[gemini-code-assist]

Code Review

This pull request updates the README documentation and pyproject.toml files across several Genkit Python packages to reflect package renaming, transition to using uv for installation, and provide updated usage examples. The review feedback correctly points out several instances where fictional or incorrect model names (such as claude-sonnet-4-6, gpt-5.2, gemini-2.5-pro, and gemini-pro-latest) were used in the example code snippets, and suggests replacing them with valid, currently available models to ensure the documentation is accurate and copy-pasteable.