fix: segfault when moving scoped_ostream_redirect (#6033)

• fix: segfault when moving scoped_ostream_redirect

The default move constructor left the stream (std::cout) pointing at the moved-from pythonbuf, whose internal buffer and streambuf pointers were nulled by the move. Any subsequent write through the stream dereferenced null, causing a segfault.

Replace = default with an explicit move constructor that re-points the stream to the new buffer and disarms the moved-from destructor.

• fix: mark move constructor noexcept to satisfy clang-tidy

• fix: use bool flag instead of nullptr sentinel for moved-from state

Using old == nullptr as the moved-from sentinel was incorrect because nullptr is a valid original rdbuf() value (e.g. std::ostream os(nullptr)). Replace with an explicit active flag so the destructor correctly restores nullptr buffers.

Add tests for the nullptr-rdbuf edge case.

• fix: remove noexcept and propagate active flag from source

• Remove noexcept: pythonbuf inherits from std::streambuf whose move is not guaranteed nothrow on all implementations. Suppress clang-tidy with NOLINTNEXTLINE instead.
• Initialize active from other.active so that moving an already moved-from object does not incorrectly re-activate the redirect.
• Only rebind the stream and disarm the source when active.

• test: add unflushed ostream redirect regression

Cover the buffered-before-move case for scoped_ostream_redirect, which still crashes despite the current move fix. This gives the PR a direct reproducer for the remaining bug path.

Made-with: Cursor

• fix: disarm moved-from pythonbuf after redirect move

The redirect guard now survives moves, but buffered output could still remain in the moved-from pythonbuf and be flushed during destruction through moved-out Python handles. Rebuild the destination put area from the transferred storage and clear the source put area so unflushed bytes follow the active redirect instead of crashing in the moved-from destructor.

Made-with: Cursor