Python: Add incomplete details reason to AzureAIAgent error message

[moonbox3]

Motivation and Context

When errors occur processing a run of an AzureAIAgent, we surface the error message but not the incomplete_details reason if it exists.

Description

This PR adds handling for the incomplete details reason, if it exists, so it can be passed back via the error message.

• Closes Python: New Feature: Surface thread run incomplete_details for Safety Evaluation in AzureAIAgent Failures #12752

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[moonbox3]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
agents/azure_ai
agent_thread_actions.py	416	193	53%	163, 165, 202–208, 244–245, 248–252, 254–256, 260, 263–265, 274, 279–280, 342, 346, 351, 354, 357–358, 362, 365, 368–369, 373, 376, 379–380, 384–385, 389–390, 394–395, 495, 497, 537–538, 560, 562–564, 566–568, 570–571, 573–578, 580–588, 592–593, 596–599, 603, 606–607, 610–611, 614, 617, 620–621, 624–628, 630–631, 635, 638–639, 646–647, 652, 656–657, 659–660, 667, 669, 673, 676–681, 683–685, 690–691, 694–695, 698–700, 710–711, 718, 720, 724, 726–727, 731–740, 742–750, 801, 804, 838–839, 846–850, 910, 915–920, 948–951, 955, 960–961, 965, 982–984, 988–989, 1004–1005, 1043, 1084–1087, 1089–1090, 1094, 1098–1099, 1104
TOTAL	26701	4600	82%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
3659	22 💤	0 ❌	0 🔥	2m 0s ⏱️