Add inbox event support and alias inbox/start for Marketplace messaging

[Daniel-Ric]

Motivation

• Provide a clearer, user-facing inbox API surface for Marketplace messaging while retaining the existing legacy path for compatibility.
• Allow clients to mark messages as seen (Impression) or delete them via a simple server-side proxy to the Marketplace messaging API.
• Keep request/response semantics consistent with other Marketplace helpers by reusing x-mc-token handling and session headers.

Description

• Added new messaging routes POST /messaging/inbox/start (keeps legacy /messaging/session/start) and POST /messaging/inbox/event with Joi validation and JWT protection in src/routes/messaging.routes.js.
• Implemented buildMarketplaceMessageEventsPayload and sendMarketplaceMessageEvents in src/services/minecraft.service.js, and extended startMarketplaceMessagingSession to support session header passthrough and request payload creation to the Marketplace endpoints.
• Wired the messaging router into the app at app.use('/messaging', messagingRoutes) and added the Messaging tag to the OpenAPI spec in src/utils/swagger.js.
• Documented endpoints in README.md and added unit tests tests/messaging.test.js for event payload construction alongside existing session payload tests.

Testing

• Ran the test suite with npm test (which runs node --test) and observed 4 passing tests and 0 failures.
• No automated test failures were introduced by the changes.

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Codex Task