Python: Fix DurableState schema serializations

[larohra]

Motivation and Context

This PR addresses the following bugs -

• Consolidating serialization for DurableState schema to use constants.
• Fix the created_at bug where the timestamp for each response content was always getting updated for each reply.
• Added a comment for the Streaming example.

Description

Fixes #2483 and #2644

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.

[markwallace-microsoft]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/azurefunctions/agent_framework_azurefunctions
_constants.py	61	0	100%
_durable_agent_state.py	458	131	71%	75, 82–83, 103–104, 130–132, 153–154, 160–161, 168–169, 176–177, 183–184, 187–188, 193–196, 202–205, 210–211, 214–215, 246, 257, 275, 279, 281, 283, 285, 288–294, 337, 400, 411–412, 421–424, 426, 452–453, 456, 458, 463, 522–523, 525, 583, 655, 666, 737, 739, 816, 819, 841–842, 845, 853, 856, 883, 897, 920–922, 925, 935–939, 941–944, 946, 949, 970–971, 974, 982, 985, 1003, 1006, 1010, 1013, 1032, 1035, 1044, 1047, 1089, 1092, 1096, 1099, 1119–1120, 1123, 1131, 1134, 1169, 1174–1176, 1180, 1191, 1199, 1222, 1225, 1232, 1235, 1254, 1257, 1261, 1264–1266
TOTAL	16274	2536	84%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
2321	130 💤	0 ❌	0 🔥	55.528s ⏱️

[Copilot]

Pull request overview

This PR consolidates and fixes serialization for the DurableState schema by introducing centralized constants, fixing a created_at timestamp bug, and improving documentation for streaming examples.

Key changes:

• Introduces DurableStateFields, ContentTypes, and ApiResponseFields constant classes to eliminate hardcoded field names and ensure consistency across serialization/deserialization
• Fixes the created_at bug where DurableAgentStateMessage.from_run_request was incorrectly defaulting to current time instead of preserving None when no timestamp was provided
• Moves DurableAgentStateEntryJsonType enum to the top of the file for better code organization
• Updates streaming example documentation to clarify in-memory store limitations and suggest distributed alternatives

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
python/packages/azurefunctions/agent_framework_azurefunctions/_constants.py	Adds comprehensive constant classes for JSON field names, content type discriminators, and API response fields to centralize serialization constants
python/packages/azurefunctions/agent_framework_azurefunctions/_durable_agent_state.py	Replaces all hardcoded strings with constants, fixes created_at handling in DurableAgentStateMessage.from_run_request, moves enum definition, and updates correlation_id type to allow None in DurableAgentStateResponse
python/packages/azurefunctions/tests/test_entities.py	Adds tests to verify created_at field handling in DurableAgentStateMessage.from_run_request with both None and valid timestamp values
python/samples/getting_started/azure_functions/03_callbacks/function_app.py	Clarifies that the in-memory callback store is for streaming examples and suggests Redis or Service Bus for distributed environments
python/samples/getting_started/azure_functions/03_callbacks/README.md	Expands documentation to explain streaming nature of the example and recommends distributed alternatives for production use

[cgillum]

Some comments below.

Also, is the python/uv.lock change required? I can't view it in GitHub because it's too big, and I worry that it will create merge conflict headaches. Please remove that change if it's not required.

[cgillum]

We should add a note here that changing the value of any of these constants is a breaking change.

[cgillum]

This last sentence is a little weird. Consider removing it since I think it's mostly adding misinformation.