Support streams with optional endpoints

[OskarEichler]

Rackup::Stream.new currently evaluates an undefined Buffered constant before it can construct the stream. Supplying only an input works around that constant but then forces a writable output, so the nominally optional endpoints are not actually optional. If input is absent, the first read should report EOF; an explicit close_read should still make later reads raise.

Make both endpoints optional, retain validation when output is present, distinguish initial EOF from explicit read closure, and make empty? safe without output.

This ports the relevant source-origin Stream corrections from socketry/protocol-http commit 2c3115e, whose Stream implementation Rackup shares.

Verification:

• deterministic endpoint/closure model fails on 2.3.1/current and passes this branch
• full suite: 76 runs / 194 assertions / 0 failures
• Ruby syntax, warning-mode load, package build and diff checks pass on Ruby 4.0.6
• combined candidate also passes 74 runs / 182 assertions on Ruby 3.2.11

No breaking change for supplied endpoints; invalid non-writable outputs remain rejected.