Add generic TCP connection handler hook

[abnegate]

Summary

• remove PostgreSQL/MySQL protocol handling from the generic TCP proxy core
• add a generic coroutine connection handler hook for callers that need protocol negotiation before routing
• expose TCPAdapter::connect() so custom handlers can reuse proxy backend dialing/socket options after resolving a route

Verification

• composer lint
• composer check
• composer test

[Copilot]

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[greptile-apps]

Greptile Summary

This PR replaces the hardcoded PostgreSQL SSLRequest interception in both the event-driven and coroutine TCP servers with a generic connectionHandler hook in Config, letting protocol-aware callers implement their own negotiation before the proxy routes a backend. The TLS class is stripped of all protocol-detection constants and helpers, and TLSContext gains toSwooleProtocolConfig() for mid-connection socket upgrades.

• Coroutine.php: listener is now always plaintext; connectionHandler is invoked per connection and may return true to take full ownership, otherwise the standard recv/forward loop proceeds. Adds startTLS() (protected helper for subclasses), stats(), shutdown(), and a connection counter wrapped in finally.
• TCP.php: backend-dialing logic extracted into a new public connect() method so handlers that resolve routing themselves can reuse the same connection-cache path as getConnection().
• TLS.php / Connection.php: pendingTls, PG_SSL_REQUEST, PG_SSL_RESPONSE_*, MYSQL_CLIENT_SSL_FLAG, and the two detection helpers are removed; their protocol-specific tests are replaced by config/reflection tests and a source-string assertion in TLSTest.php.

Confidence Score: 5/5

Safe to merge; the architectural shift is clean and consistent across both server implementations.

The core forwarding and TLS-upgrade paths are correctly refactored. The removed SSLRequest handling in the event-driven server was unused in practice (Swoole's SWOOLE_SSL mode does TLS immediately on accept, before any PostgreSQL wire bytes arrive). The only open items are documentation and test-quality nits that do not affect runtime correctness.

tests/TLSTest.php relies on source-code string matching rather than runtime assertions; src/Server/TCP/Config.php and src/Server/TCP/Swoole/Coroutine.php would benefit from a docblock describing the connectionHandler return-value contract and the accessibility of startTLS().

Important Files Changed

Filename	Overview
src/Server/TCP/Swoole/Coroutine.php	Refactored to always use a plaintext listener; PostgreSQL SSLRequest handling removed in favour of a generic connectionHandler hook. Adds connection counter, GC timer tracking, stats(), startTLS() helper (protected, not callable from handler closures), and shutdown().
src/Server/TCP/Config.php	Adds connectionHandler closure property with no docblock describing the required return-value contract.
src/Adapter/TCP.php	Extracts backend-dialing logic from getConnection() into a new public connect() method so protocol-aware handlers can route first then reuse the same connection-cache path.
src/Server/TCP/TLS.php	Removes PostgreSQL/MySQL protocol constants and detection helpers that are now the caller's responsibility.
src/Server/TCP/TLSContext.php	Adds toSwooleProtocolConfig() for mid-connection TLS upgrade via Socket::setProtocol() + Socket::sslHandshake(); cleanly separates immediate-accept TLS from negotiate-then-upgrade TLS.
src/Server/TCP/Swoole.php	Removes PostgreSQL STARTTLS interception from onReceive; adds a stats() helper wrapping Server::stats() with safe key normalisation.
src/Server/TCP/Connection.php	Drops the pendingTls flag that is no longer needed after removing inline SSLRequest handling.
tests/TLSTest.php	Replaces protocol-detection unit tests with source-code string assertions and lightweight config/reflection tests; one new test inspects raw PHP source bytes rather than runtime behaviour.
tests/TLSContextTest.php	Adds a focused unit test for toSwooleProtocolConfig() verifying open_ssl is set alongside cert/key paths.
tests/ConnectionStructTest.php	Removes pendingTls assertions to match the dropped field in Connection.

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