.NET: Python: Add AGENTS.md files and update coding standards (#3644)
* Add AGENTS.md files and update coding standards for Python
- Add root python/AGENTS.md with project structure and package links
- Add AGENTS.md for each package describing purpose and main classes
- Update .github/copilot-instructions.md with improved structure
- Update python/CODING_STANDARD.md with API review guidance:
- Future annotations convention (#3578)
- TypeVar naming convention (#3594)
- Mapping vs MutableMapping (#3577)
- Avoid shadowing built-ins (#3583)
- Explicit exports (#3605)
- Exception documentation guidelines (#3410)
- Simplify python/.github/instructions/python.instructions.md to reference AGENTS.md
- Remove AGENTS.md from .gitignore
* Fix purview import path in AGENTS.md
* Address PR review comments and restructure instructions
- Slim down .github/copilot-instructions.md to reference language-specific docs
- Add ADR section explaining templates and purpose
- Create dotnet/AGENTS.md with .NET-specific build commands, conventions, and sample guidance
- Update Python build/test instructions for core vs isolated changes
- Fix Microsoft.Extensions.AI package references
- Update kwargs guidance per issue #3642
- Fix Python sample helper placement (top, not bottom)
- Document new 'typing' poe task in DEV_SETUP.md
* Add 'typing' poe task to run both pyright and mypy
* Add kwargs guidelines from issue #3642 to CODING_STANDARD.md
* Clarify that connector packages pull in core as dependency
Eduard van Valkenburg