.NET Workflows - Rename agent-provider and add comments (Declarative Workflows)

[crickman]

Motivation and Context

Rename the WorkflowAgentProvider contract to that its clear that it has an affinity for Response API patterns.

Description

@alliscode and @markwallace-microsoft proposed WorkflowAgentProvider be renamed to highlight that Declrative Workflows only works with Response API (including Foundry V2 Agents). At some point in the future, generalizing this contract for any AIAgent might be addressed.

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.

[Copilot]

Copilot reviewed 19 out of 19 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/ResponseAgentProvider.cs:17

• The phrasing "is very much opinionated" is awkward. Consider simplifying to "is opinionated" or "is strongly opinionated" or "heavily favors" for more natural English.
  dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/ResponseAgentProvider.cs:14
• The phrase "Defines contract used by" is missing an article. It should be "Defines the contract used by" or "Defines a contract used by" for grammatically correct English.