Python: make declarative YAML path loading UTF-8 and async-safe

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Motivation & Context

Declarative YAML path loaders relied on the process locale, so UTF-8 definitions containing characters outside a platform's default encoding could fail before parsing. The async agent path loader also performed filesystem reads directly on the event-loop thread.

Description & Review Guide

• What are the major changes? Agent and workflow YAML path loaders now read files explicitly as UTF-8. The async agent loader performs the read with asyncio.to_thread() and translates a missing file back to DeclarativeLoaderError.
• What is the impact of these changes? UTF-8 declarative definitions load consistently across platforms, and asynchronous agent construction no longer blocks the event loop during file access.
• What do you want reviewers to focus on? Please review the missing-file exception compatibility and whether UTF-8 is the expected portable encoding for declarative definition files.

Related Issue

Fixes #8254

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 a language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Copilot]

🟢 Approval recommended

The focused implementation preserves missing-file behavior and adequately tests UTF-8 and asynchronous file access.

Pull request overview

Makes declarative YAML path loading portable and async-safe.

Changes:

• Reads agent and workflow YAML files explicitly as UTF-8.
• Offloads asynchronous agent file reads with asyncio.to_thread().
• Adds regression tests for Unicode content, threading, and encoding.

File summaries

File	Description
_loader.py	Adds UTF-8 agent loading and non-blocking async reads.
_workflows/_factory.py	Adds explicit UTF-8 workflow loading.
test_declarative_loader.py	Tests agent encoding and async thread offloading.
test_workflow_factory.py	Tests workflow UTF-8 loading.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 0
• Review effort level: Balanced

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[Evan Mattson (moonbox3)]

Please agree to the CLA

[fzfzzfzzzfzzzz]

@microsoft-github-policy-service agree

[Eduard van Valkenburg (eavanvalkenburg)]

Thanks for the update. Before this is ready, could you please:

• get the non-green checks passing: merge-gatekeeper (FAILURE), command_check (CANCELLED), team_check (CANCELLED), review (CANCELLED)

Once those are addressed, please re-request review. Thanks!

[fzfzzfzzzfzzzz]

Thanks! I checked the failed checks. The CLA is green now.

The DevFlow PR Review run was cancelled by the workflow's concurrency handling (Canceling since a higher priority waiting request ... exists), so command_check and team_check never completed successfully. I don't have permission to rerun the upstream workflow as an external contributor.

Could you please retrigger the DevFlow PR Review for #8260? Once command_check and team_check pass, the Merge Gatekeeper can be rerun and I'll re-request review.