Python: fix declarative workflow DevUI message input

[Xiaoping Liao (kyletser)]

Motivation & Context

The declarative DevUI sample receives chat input as a single Message, while the entry JoinExecutor only accepts list[Message]. The workflow state therefore does not populate System.LastMessage.Text from the DevUI input. The sample also reads a structured inputs.age value and uses expression namespaces that do not match the Python runtime, so every age follows the child branch.

Description & Review Guide

• What are the major changes? Treat a single Message like a one-item message list during declarative state initialization. Update the sample to parse System.LastMessage.Text with the existing ParseValue action, store per-run values under Local.*, and write the result to Workflow.Outputs.*.
• What is the impact of these changes? DevUI chat input now reaches the workflow as the expected age value. Existing dict, string, and list[Message] inputs keep their current behavior.
• What do you want reviewers to focus on? Please check the single-message state initialization and the use of ParseValue for the sample. Regression coverage includes direct state assertions and the child, teenager, adult, and senior branches.

Related Issue

Fixes #7285

#7206 attempted to fix the same issue but was closed with an unresolved concern about a global Int() fallback. This version uses the existing ParseValue action and does not add or override Power Fx conversion behavior.

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.

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[Xiaoping Liao (kyletser)]

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