test: refactor StreamSocketClient tests into focused test cases

[guillaumedelre]

Split the two existing test files (Tcp and Udp) into four focused test cases as described in issue #112:

• StreamSocketClientTest: general protocol-agnostic tests extracted from the TCP file (context management, timeout, socket lifecycle, invalid constructor arguments)
• StreamSocketClientTcpTest: TCP-specific data transfer tests only (write, bad write, multi-write, read, close behaviors)
• StreamSocketClientUdpTest: trimmed to UDP-specific write test only, removing duplicates now covered by the general test case
• StreamSocketClientSslTest: new test case covering SSL failure path (connecting with ssl:// to a plain TCP server) and SSL context preservation through the constructor

Fix three pre-existing Psalm errors surfaced during this work:

• Logger.php: add @param docblock for $message (string|\Stringable) without touching the PHP signature, which must stay untyped for psr/log ^1 compatibility (see gelf-php 2.0.0 incompatible with psr/log:^1 due to method signature being too strict #151)
• Message.php: add explicit mixed type and @psalm-suppress on the empty() call inside the array_filter closure to preserve the existing behavior
• HttpTransport.php: replace truthy check on null|string with strict null and empty-string comparisons in fromUrl()

Closes #112

[guillaumedelre]

Hey @bzikarsky, this PR fixes #112 — happy to adjust if anything looks off.