Python: Bug fixes for devui serialization and agent structured outputs

[Evan Mattson (moonbox3)]

Motivation and Context

While testing, was running into two issues:

1. Devui was failing to handle serializing an agent-related event. Needs a serialization helper (can later look to streamline in workflows as well)
2. Structured outputs are getting lost when merging chat options because the pydantic class isn't serializable. Now we hold on to it, if configured, and reset it to response_format after merging chat options.

Devui now passes as we're handling structured outputs throughout the workflow/agent calls:

Description

Bug fixes

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? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull Request Overview

This PR fixes two serialization-related bugs in the agent framework: DevUI failing to serialize agent-related events and structured outputs being lost during chat options merging.

• Added a comprehensive payload serialization helper for DevUI to handle complex objects
• Fixed structured output preservation during ChatOptions merging operations

Reviewed Changes

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

File	Description
python/packages/devui/agent_framework_devui/_mapper.py	Added _serialize_payload method to handle serialization of workflow event data
python/packages/core/agent_framework/_types.py	Fixed response_format preservation in ChatOptions merging logic

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[Mark Wallace (markwallace-microsoft)]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/core/agent_framework
_types.py	881	93	89%	126–127, 145–146, 253, 255, 262, 281, 321, 357–358, 400, 536, 633–634, 636, 661, 668, 685–687, 743, 748–749, 751, 758–759, 761, 783, 790, 793–795, 800–801, 807–809, 910, 1007, 1138, 1142, 1144, 1323–1325, 1331–1332, 1491, 1496, 1531, 1536, 1540, 1544, 1713–1717, 1727, 1732, 1941, 2072, 2077, 2140–2142, 2165, 2185, 2201, 2209–2213, 2287, 2336, 2340, 2348, 2352–2354, 2415, 2419, 2423, 2427, 2430, 2484–2486, 2554, 2581, 2588
TOTAL	10117	1796	82%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
1174	95 💤	0 ❌	0 🔥	30.938s ⏱️