Python: Improve Azure AI Search package test coverage

[TaoChenOSU]

Motivation and Context

Closes #3354

Description

1. Increase Azure AI Search unit test coverage to 97%
2. Enforce coverage stay above 85%

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
TOTAL	20606	2987	85%

report-only-changed-files is enabled. No files were changed during this commit :)

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
4087	225 💤	0 ❌	0 🔥	1m 17s ⏱️

[Copilot]

Pull request overview

This PR significantly improves the test coverage for the Azure AI Search package, achieving 97% coverage and enforcing a minimum 85% threshold through CI. The changes address issue #3354 by adding comprehensive unit tests for previously untested functionality in the AzureAISearchContextProvider class.

Changes:

• Added extensive unit tests covering initialization, credential resolution, async context manager, vector field discovery, semantic search, agentic search, and document text extraction
• Enabled coverage enforcement for the azure-ai-search package in the CI pipeline to maintain quality standards

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
python/packages/azure-ai-search/tests/test_aisearch_context_provider.py	Added 880+ lines of comprehensive unit tests covering edge cases, error handling, and various configuration scenarios for the AzureAISearchContextProvider class
.github/workflows/python-check-coverage.py	Added azure-ai-search package to the enforced modules list, requiring it to maintain at least 85% test coverage in CI