Add a short guide for testing idempotency and conflict handling

[MuttleyD]

Context

MIDAS supports idempotent runtime evaluation using request_source and request_id. This is an important part of the runtime contract because callers need to know what happens when they retry the same decision request.

The control-plane example walkthrough already validates idempotent replay and changed-payload conflict behaviour, but this behaviour should be easier for new users to understand.

Scope

Add a concise idempotency and conflict testing section to docs/examples/control-plane.md.

The section should explain:

• request_source and request_id form the scoped idempotency key;
• replaying the same request with the same payload should return the same decision/envelope;
• reusing the same request_source and request_id with a changed payload should return 409 Conflict;
• this protects callers from accidental duplicate decision records while still detecting conflicting retries.

Include example commands showing:

• first /v1/evaluate request;
• exact replay of the same request;
• changed-payload request using the same request_source and request_id;
• expected 409 response.

Acceptance Criteria

• docs/examples/control-plane.md explains idempotent replay in plain language.
• The section includes copyable example commands.
• The examples use one of the existing control-plane bundles and validated request IDs.
• The expected replay and conflict behaviours match the current runtime.
• The section links to docs/core/runtime-evaluation.md for deeper details.
• No runtime behaviour is changed.
• No real secrets are added.
• git diff --check passes.

Out of Scope

• Changing idempotency semantics.
• Changing runtime evaluation behaviour.
• Adding new endpoints.
• Adding automation or scripts.
• Changing the example bundles.