gh-124293: Fix tar stream seekability checks - #159854

Implement seekable() on the tar stream adapter so extractfile() results support seekability checks and forward seeks without raising AttributeError.

Update the attribute checks and add a forward-seek regression inherited by the compressed stream test classes. Stream reads still reject attempts to access data that has already been passed.

Fixes #124293.

Validation:

• The new forward-seek regression raises AttributeError before the fix.
• Full tarfile suite on a CPython debug build: 795 tests, 113 skipped.
• Ruff, NEWS lint, and patchcheck.

• Issue: '_Stream' object has no attribute 'seekable' #124293