Fix SocketHandler firing writingTimeout instantly instead of waiting

[wakqasahmed]

SocketHandler::writeToSocket() resets $lastSentBytes to 0 at the top of every call, but never touches $lastWritingAt, which starts out as null. If the very first fwrite() in a call makes zero progress, writingIsTimedOut() sees $sent === $lastSentBytes (both 0) and takes the stalled branch, then compares microtime(true) against (float) $this->lastWritingAt. Since that's still null, it casts to 0.0, so the elapsed-time check is always true on the very first stall — the socket gets closed and a timeout exception is thrown in a fraction of a millisecond, instead of after the configured writingTimeout seconds.

There's already a test (testAvoidInfiniteLoopWhenNoDataIsWrittenForAWritingTimeoutSeconds) that mocks fwrite to always return 0, but it only asserts that a RuntimeException is eventually thrown, not when — so it's been passing the whole time despite the timeout never actually being honored.

Fix is a one-line initialization: set $lastWritingAt = microtime(true) alongside the existing $lastSentBytes reset, so the first stall check has a real baseline to measure from.

Added a new test that mocks the same always-stalled fwrite/streamGetMetadata scenario but asserts on timing: it sets a short writingTimeout, times how long writeToSocket takes to throw, and checks that the elapsed time is actually close to the configured timeout rather than near-zero. I verified this test fails against the current code (throws in ~0.0007s against a 0.3s timeout) and passes with the fix applied.

Ran the full SocketHandlerTest suite locally (PHP 8.3): 21 passed, 0 failed.