Python: Handle missing reasoning text in DevUI

[ZedingZhang (ZedingZhang)]

Motivation & Context

DevUI turns a valid reasoning chunk with text=None into a ResponseReasoningTextDeltaEvent validation error and can discard the answer following it in the same update.

Description & Review Guide

• What are the major changes? Map missing reasoning text to an empty string at the existing DevUI event boundary. Add a parametrized regression using real framework content for None, empty text, and ordinary reasoning text.
• What is the impact of these changes? Reasoning events remain valid and the following answer is delivered without a conversion warning. The production fix is one line.
• What do you want reviewers to focus on? The optional-text/string-event boundary and continuation of the same update. Ablation retained only the inline fallback: both explicit normalization and the one-line form pass; removing the fallback reproduces the reported validation error while the two string controls pass.

Related Issue

Fixes #9533.

Trivial-change exception: this is an obvious correction of a valid optional value being passed to a string-only field, using the one-line fallback already proposed in the issue. The existing event contract is preserved.

AI Assistance

• No material AI assistance was used.
• This is an AI-assisted contribution.

AI assistance details: OpenAI Codex prepared the implementation, regression tests, ablation experiments, and this description, and ran the local checks. Two independent Codex review contexts checked Standards and Spec. I have reviewed all submitted content and take resonsibility for it.

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible — affected DevUI package: 217 passed, 4 skipped; package checks and test typing passed.
• The PR follows the Contribution Guidelines
• This PR links to an agreed issue with no competing open PR, or the Related Issue section documents a trivial-change or repository-automation exception.
• 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.

[Copilot]

🟢 Approval recommended

The localized fix preserves the event contract, and regression coverage addresses the reported failure with no identified issues.

No open issues 🎉

What changed in this PR

Fixes #9533 by allowing DevUI to handle reasoning chunks with missing text without dropping the following answer.

Changes:

• Converts missing reasoning text to an empty string at the event boundary.
• Adds regression coverage for missing, empty, and ordinary reasoning text, including answer continuation.

File	Description
python/​packages/​devui/​tests/​devui/​test_mapper.py	Tests reasoning deltas, answer delivery, and absence of conversion warnings.
python/​packages/​devui/​agent_framework_devui/​_mapper.py	Adds the empty-string fallback for reasoning text.

_{Review effort: Balanced}