feat(api): return the target session from POST /mocks

[gwleclerc]

Relates to #312.

What

POST /mocks now returns the target session (id + name + date) alongside the existing message:

{
  "message": "Mocks registered successfully",
  "session": { "id": "W0dArqgEPE4m", "name": "my-suite", "date": "..." }
}

Why

Registering mocks (optionally creating a session via ?newSession=<name>) previously returned only a message, with no session id — so a caller, especially a shell script, had no handle to then fetch the mocks it just registered. It had to GET /sessions and guess by name, which isn't reliable (session names need not be unique; the opaque id is the key — see PRINCIPLES.md §3.6).

Now the flow is a clean chain:

sid=$(curl -s -XPOST "$SMOCKER/mocks?newSession=my-suite" --data-binary @mocks.yml | jq -r .session.id)
curl -s "$SMOCKER/mocks?session=$sid"

Compatibility

Purely additive — the message field is unchanged, so existing clients are unaffected.

Verified

• Unit + golden/schema tests green; a venom assertion covers session.id / session.name in the response.
• Runtime: POST /mocks?newSession=my-suite returns the session id, and GET /mocks?session=<id> returns the registered mock.

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