Add Python-wide code ownership

[eavanvalkenburg]

Motivation & Context

Ensure Python ownership is consistent across the repository so @eavanvalkenburg is requested for reviews throughout the Python codebase, including paths with package-specific overrides.

Description & Review Guide

• What are the major changes? Added @eavanvalkenburg to the five Python-specific CODEOWNERS rules that previously overrode the broader /python rule without including that owner.
• What is the impact of these changes? Changes under ag-ui, chatkit, core workflows, declarative, and orchestrations will now include @eavanvalkenburg as a code owner.
• What do you want reviewers to focus on? Confirm that every Python CODEOWNERS pattern now includes @eavanvalkenburg.

Related Issue

N/A — repository ownership maintenance requested directly.

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[eavanvalkenburg]

Superseded by microsoft#7915.