chore: remove support for shell and docker workflow executors

Description

Removes support for the shell and docker executors.

This will help resolve confusion and reduce customer settings when using the agentic platform in the IDEs.

This MR cleans up all configuration reading/passing within VSCode, and means the language server will always use its default values (node executor).

Related Issues

Resolves #2080 (closed)

How has this been tested?

• Start the development extension host with this branch
• Configure your VSCode settings.json to use one of the previous executors, e.g.

 "gitlab.duo.workflow.useDocker": true,
or
 "gitlab.duo.workflow.executor": "shell",
or
 "gitlab.duo.workflow.executor": "docker",

• Start an agentic chat or flow

• View the language server debug logs

• Notice it ignores your extension configuration and uses the NodeExecutor to run the chat/flow

• If src/browser or src/common has been modified, please consider interoperability with the Web IDE. See Running the Extension in WebIDE.

• Consider an end-to-end test for significant new features that aren't covered by integration tests.

Screenshots (if appropriate)

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes