FEAT: Add stress test pipeline and migrate it to new 1ES Pool/Images (#432)

### Work Item / Issue Reference  
<!-- 
IMPORTANT: Please follow the PR template guidelines below.
For mssql-python maintainers: Insert your ADO Work Item ID below 
For external contributors: Insert Github Issue number below
Only one reference is required - either GitHub issue OR ADO Work Item.
-->

<!-- mssql-python maintainers: ADO Work Item -->
>
[AB#41877](https://sqlclientdrivers.visualstudio.com/c6d89619-62de-46a0-8b46-70b92a84d85e/_workitems/edit/41877)

<!-- External contributors: GitHub Issue -->
> GitHub Issue: #<ISSUE_NUMBER>

-------------------------------------------------------------------
### Summary   
<!-- Insert your summary of changes below. Minimum 10 characters
required. -->
This pull request adds a comprehensive suite of multi-threaded stress
tests for the `mssql-python` package. The new tests cover a wide range
of concurrency scenarios, sustained load, pool lifecycle, and heavy
query scaling to ensure robust performance and stability under high-load
conditions. The tests are cross-platform, feature deterministic
shutdowns, and include adaptive validation of error rates.

Concurrency and load testing:

* Added a concurrency matrix of tests (`test_concurrency_matrix`) that
runs queries with varying thread counts and connection pooling settings,
validating error rates at different concurrency levels.
* Introduced sustained load tests (`test_sustained_load`) that run
multiple threads for extended durations to simulate real-world high-load
situations.
* Implemented heavy query scaling tests (`test_heavy_query_scaling`)
with large result sets and high thread counts to stress test the
connection pool.

Connection pool management:

* Added a pool lifecycle test (`test_pool_lifecycle_cycles`) to
repeatedly enable and disable connection pooling, verifying
deterministic shutdown and resource cleanup.
* Provided utilities for strict thread joining and pool shutdown to
ensure clean test teardown and avoid resource leaks.

Test infrastructure and utilities:

* Defined data models (`ThreadResult`, `StressResult`) and a runner
class (`MultiThreadedQueryRunner`) for collecting detailed metrics and
managing

<!-- 
### PR Title Guide

> For feature requests
FEAT: (short-description)

> For non-feature requests like test case updates, config updates ,
dependency updates etc
CHORE: (short-description) 

> For Fix requests
FIX: (short-description)

> For doc update requests 
DOC: (short-description)

> For Formatting, indentation, or styling update
STYLE: (short-description)

> For Refactor, without any feature changes
REFACTOR: (short-description)

> For release related changes, without any feature changes
RELEASE: #<RELEASE_VERSION> (short-description) 

### Contribution Guidelines

External contributors:
- Create a GitHub issue first:
https://github.com/microsoft/mssql-python/issues/new
- Link the GitHub issue in the "GitHub Issue" section above
- Follow the PR title format and provide a meaningful summary

mssql-python maintainers:
- Create an ADO Work Item following internal processes
- Link the ADO Work Item in the "ADO Work Item" section above  
- Follow the PR title format and provide a meaningful summary
-->

Author: gargsaumya