Resolve flow context in BuildAdditionalContextService

What does this MR do and why?

The service now takes the foundational flow and appends the envelopes its additional_context_resolver returns. Every start path reaches this service, so a REST-started session gets the same context as a trigger-started one.

Drops the duplicate resolver calls in CreateAndStartWorkflowService and FlowTriggers::RunService, and the bespoke resource context envelope that RunService built.

For a flow that owns a resolver, a caller-supplied additional_context is now additive: a colliding category loses to the generated envelope.

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