#16006: fix(gateway): reduce chat.history byte cap from 6 MB to 2 MB

## Summary - Lower `DEFAULT_MAX_CHAT_HISTORY_MESSAGES_BYTES` from 6 MiB to 2 MiB - Sessions with large transcripts (~6 MB+) crash the browser tab because `chat.history` sends the full capped payload in a single WebSocket frame - 2 MiB is still plenty for ~200 messages of normal conversation while staying within what browsers can comfortably parse and render ## Test plan - [x] `session-utils.test.ts` (34 tests) — all pass, `capArrayByJsonBytes` uses custom limits unaffected by default change - [x] Build + lint clean Fixes #15992 lobster-biscuit > 🤖 Generated with [Claude Code](https://claude.com/claude-code) <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Reduced `DEFAULT_MAX_CHAT_HISTORY_MESSAGES_BYTES` from 6 MiB to 2 MiB to prevent browser tab crashes when sending large chat history transcripts via WebSocket. The change addresses sessions with ~6 MB+ transcripts that were causing browser tabs to freeze when `chat.history` sent the full payload in a single WebSocket frame. 2 MiB still provides capacity for approximately 200 messages while staying within browser parsing limits. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk - The change is a simple constant value reduction that addresses a real browser performance issue. The new value (2 MiB) is still generous for typical use cases (~200 messages), and the `capArrayByJsonBytes` function already handles byte capping correctly. Tests pass and the change is well-justified in both the PR description and issue reference. - No files require special attention _{Last reviewed commit: 4d3bcde} <!-- greptile_other_comments_section --> _{(5/5) You can turn off certain types of comments like style [here](https://app.greptile.com/review/github)!} <!-- /greptile_comment -->