Python: Add core types and agents unit tests

[giles17]

Motivation and Context

This PR adds unit tests to improve coverage for the core _types.py and _agents.py modules as part of issue #3356 (achieving 85-90% unit test coverage for agent-framework-core).

Description

Added comprehensive unit tests for:

_types.py (82% → 85%)

_agents.py (81% → 86%)

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• [] Is this a breaking change? No

[markwallace-microsoft]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
TOTAL	16249	2024	87%

report-only-changed-files is enabled. No files were changed during this commit :)

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
3674	221 💤	0 ❌	0 🔥	1m 4s ⏱️

[Copilot]

Pull request overview

Adds new unit tests to improve coverage for agent_framework core modules, focusing on content handling/validation utilities and agent option/name logic to support the coverage goals in issue #3356.

Changes:

• Added tests for Content behaviors and internal helpers in _types.py (usage aggregation, media type detection, argument parsing, data URI decoding, content list parsing, URI validation).
• Added tests for _agents.py internals and edge cases (option merging, agent name sanitization, ChatAgent init validation, thread creation).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
python/packages/core/tests/core/test_types.py	Adds coverage for Content operations and _types helper functions around parsing/validation and data URIs.
python/packages/core/tests/core/test_agents.py	Adds coverage for agent option merging/name sanitization and ChatAgent initialization/thread creation edge cases.