fix: align Slack pagination with chat sdk and raise pg limits

[Zygimantass]

Summary

• update the @chat-adapter/slack patch to match current upstream #601 task-to-segment routing behavior
• update Slackbot v2 emulator expectations for upstream STREAM_TASK_LIMIT=50
• raise the api-rs SQLx session pool cap to 500 and configure bundled Postgres with max_connections=500

Investigation

• linked thread slack:C0A87C21805:1781113111.619999 completed in DB and Slack had visible completed bot replies
• live pg_stat_activity showed the api-rs pod holding exactly 50 idle connections while Postgres was still capped at max_connections=100
• no direct too many clients log was captured for that exact thread, but the configured limits matched the suspected bottleneck

Validation

• cargo fmt --all
• cargo test -p centaur-session-sqlx
• cargo test -p centaur-api-server
• cargo build -p centaur-api-server --release
• pnpm --filter slackbotv2 test
• pnpm --filter slackbotv2 check:types
• helm dependency build contrib/chart
• helm template centaur contrib/chart