Python: Add factory pattern to sequential orchestration builder

[Tao Chen (TaoChenOSU)]

Motivation and Context

Follow up for #2486

Description

This PR adds factory to the sequential orchestration builder.

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.

[Mark Wallace (markwallace-microsoft)]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/core/agent_framework/_workflows
_sequential.py	108	15	86%	72, 76, 86, 182, 188, 234, 236–238, 253, 260, 288, 292–293, 306
_typing_utils.py	107	28	73%	56, 80, 159, 161–162, 171, 173, 180, 182, 202, 204, 206, 211–218, 221–222, 224–228, 230
TOTAL	16494	2571	84%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
2355	130 💤	0 ❌	0 🔥	58.208s ⏱️

[Copilot]

Pull request overview

This PR adds a factory pattern to the SequentialBuilder to enable proper state isolation between workflow instances. The feature allows users to register participant factories instead of participant instances, ensuring that each workflow built from the same builder gets fresh participant instances with independent state.

Key Changes:

• Added register_participants() method to SequentialBuilder accepting factory functions
• Updated SequentialBuilder to call factories during build() to create fresh participant instances
• Added comprehensive test coverage for factory pattern with various scenarios

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
python/samples/getting_started/workflows/orchestration/sequential_participant_factory.py	New sample demonstrating factory pattern usage with stateful accumulator showing state isolation between workflow instances
python/samples/getting_started/workflows/orchestration/sequential_custom_executors.py	Removed unused Never import and simplified code by removing type ignore comments
python/packages/core/tests/workflow/test_sequential.py	Added comprehensive tests for factory pattern including validation, checkpointing, builder reusability, and mixed participants
python/packages/core/agent_framework/_workflows/_typing_utils.py	Removed unused _coerce_to_type function and its associated imports
python/packages/core/agent_framework/_workflows/_sequential.py	Implemented factory pattern support with validation, added register_participants() method, updated build logic to call factories