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	459	130	71%	75, 82–83, 103–104, 130–132, 149, 159–160, 167–168, 176–177, 185–186, 193–194, 200–201, 207–208, 212–213, 220–223, 229, 231, 265, 276, 294, 298, 300, 302, 304, 307–313, 356, 419, 430–431, 440–443, 445, 471–472, 475, 477, 482, 541–542, 544, 602, 674, 685, 756, 758, 835, 838, 860–861, 864, 872, 875, 902, 916, 939–941, 944, 954–958, 960–963, 965, 968, 989–990, 993, 1001, 1004, 1022, 1025, 1029, 1032, 1051, 1054, 1063, 1066, 1108, 1111, 1115, 1118, 1138–1139, 1142, 1150, 1153, 1188, 1193–1195, 1199, 1210, 1218, 1241, 1244, 1251, 1254, 1273, 1276, 1280, 1283–1285
TOTAL	16283	2546	84%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
2324	130 💤	0 ❌	0 🔥	57.323s ⏱️

[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.