feat: enable TCP keepalive and nodelay on all connections (DRIVERS-383)

[samwar]

Summary

Adds {keepalive, true} and {nodelay, true} socket options to both gen_tcp:connect and ssl:connect calls in mc_worker_logic:do_connect/5.

This brings the Erlang driver into compliance with MongoDB specification DRIVERS-383, which requires all drivers to enable TCP keepalive by default. All other official MongoDB drivers (Node.js, Python, Java, Go, C) already comply.

Problem

Without TCP keepalive, idle connections are silently closed by firewalls and load balancers, causing high connection churn in production environments.

Changes

• src/connection/mc_worker_logic.erl: Added {keepalive, true}, {nodelay, true} to both the plain TCP and SSL clauses of do_connect/5
• test/tcp_keepalive_SUITE.erl: New test suite with property-based tests (PropEr, 100 iterations each) and unit tests

Socket option precedence (SSL)

For SSL connections, user-provided ssl_opts are appended after the defaults (++ Opts), so users can override keepalive/nodelay if needed (last occurrence wins).

Tests

Test	Type	Result
prop_keepalive_nodelay_enabled	Property (100 iter)	✅
prop_existing_options_preserved	Property (100 iter)	✅
prop_ssl_user_option_override	Property (100 iter)	⏭️ Skips without SSL MongoDB
unit_keepalive_concrete	Unit	✅
unit_keepalive_ping	Unit	✅

Scope

• Two lines changed in production code
• No new modules, records, or supervision tree changes
• SRV-based connections covered automatically (delegates to do_connect/5)
• Fully backward compatible — no configuration changes required

[samwar]

I've been noticing a lot of silently closed connections on long-running operations after upgrading my production nodes from MongoDB 5 to MongoDB 6. My hope is that by making the driver compliant with TCP keep alive and Mongo's own specifications that this will resolve those errors.

[samwar]

@comtihon Let me know if you see any issues with this PR.

[samwar]

@comtihon pinging you again to hopefully have this reviewed and merged.

[comtihon]

Somehow missed this. Thank you!