Roslyn's MEF-based "Generate overrides..." leaks into headless code actions and throws on apply

What

Requesting code actions on a type declaration can surface Roslyn's own Generate overrides... provider alongside the sidecar's headless replacement (HeadlessOverrideCodeAction). Resolving the Roslyn one fails:

Service of type 'Microsoft.CodeAnalysis.PickMembers.IPickMembersService' is required
to accomplish the task but is not available from 'MSBuildWorkspace' workspace.

IPickMembersService is a MEF-composed IDE service that does not exist in a headless MSBuildWorkspace, which is precisely why HeadlessOverrideCodeAction exists. Both actions carry the identical title Generate overrides..., so which one a client gets depends on ordering — the user sees an action that sometimes works and sometimes errors.

Reproduce

Observed against src/editors/vscode/test-fixtures-style sources while writing HeadlessOverrideGenerationTests. Put the caret on the identifier of an abstract class that inherits only object's virtuals (so the headless provider contributes nothing), request code actions, then resolve the Generate overrides... entry.

Expected

MEF-dependent providers that cannot run headlessly should be filtered out of the code-action list, so the only Generate overrides... offered is the one that actually works. A duplicate title should never be ambiguous.

Notes

Not a regression from the current PR — the headless replacement and the leak arrived together. Filed separately rather than widening that PR's scope.

Start with HeadlessOverrideGenerationTests and the code-action path involving HeadlessOverrideCodeAction, using the src/editors/vscode/test-fixtures-style sources described in the report. Reproduce the duplicate Generate overrides... actions on an abstract class and resolve each entry. Done means the MEF-dependent action is absent from headless results and the remaining action resolves without the IPickMembersService error.