[Copilot]

Several stream behaviors were inconsistent: the PHP input stream was not cached, StringStream ignored write restrictions and did not advance after reading its remaining contents, and a null decorator target left its inner stream uninitialized.

• Caching: Reuse the PhpInputStream instance returned by getInstance().
• String streams: Reject writes when not writable and advance the position after getContents().
• Decorator streams: Initialize a missing target with an empty Stream.
• Regression coverage: Add tests for singleton reuse, blocked writes, position advancement, and the default decorator target.

$stream = new StringStream('FOO');
$stream->getContents();

$stream->tell(); // 3

• Fixes Stream Issues #1376