#6805: fix: increase WebSocket MAX_PAYLOAD_BYTES to 6MB for attachments

## Problem `chat.send` supports attachments up to 5MB (`MEDIA_MAX_BYTES` in `src/media/store.ts`), and node devices can send base64-encoded images via the gateway WebSocket. However, `MAX_PAYLOAD_BYTES` in `server-constants.ts` was set to 512KB, which silently rejects any payload over that limit. Typical phone photos are 1-2MB raw (~1.3-2.7MB base64-encoded), well within the 5MB attachment limit but far exceeding the 512KB WebSocket frame cap. This means node devices (e.g., mobile apps sending camera photos) have their payloads silently dropped. ## Fix Bump `MAX_PAYLOAD_BYTES` from 512KB to 6MB (5MB attachment limit + JSON overhead) to align with the documented attachment size limit. ## Testing - Tested with a Flutter-based node app sending batches of up to 10 photos (each ~1.3MB base64) - All photos successfully received after the change - No impact on normal text message payloads ## Related - `MEDIA_MAX_BYTES = 5 * 1024 * 1024` in `src/media/store.ts` - `MAX_BUFFERED_BYTES = 1.5 * 1024 * 1024` (send buffer, unchanged) <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR increases the gateway WebSocket `MAX_PAYLOAD_BYTES` limit (used as `ws`’s `maxPayload`) from 512KB to 6MB so node clients can send image payloads/attachments without being dropped. The change flows through both the `WebSocketServer` configuration (`createGatewayRuntimeState`) and the `hello-ok` policy payload that tells clients the server limits. Noted issues: one existing test still hard-codes the old 512KB `policy.maxPayload`, and the new inline comment may overstate that 6MB is sufficient for base64-encoded 5MB attachments (base64 expansion can exceed 6MB). <h3>Confidence Score: 4/5</h3> - This PR is likely safe to merge, with minor correctness/documentation and test-alignment concerns. - Change is a single constant bump that aligns with existing attachment limits and is already validated by manual testing, but there’s a stale hard-coded value in a gateway client test and the accompanying comment may be misleading about base64 size expansion near the boundary. - src/gateway/server-constants.ts and src/gateway/client.test.ts <!-- greptile_other_comments_section --> <!-- /greptile_comment -->