python: identify_stream() raises UnboundLocalError instead of the real OSError on unseekable streams

Summary

Magika.identify_stream() raises UnboundLocalError instead of propagating the real I/O error when the input stream passes every type guard but its tell() fails (i.e. a readable-but-unseekable io.BufferedIOBase, such as sys.stdin.buffer on a pipe or socket.makefile("rb")). The real OSError: [Errno 29] Illegal seek is discarded as the context of a bogus internal error.

Environment

• main @ e6a4c8e ("Expose the Rust library as a C library (#1449)")
• python/ package version 1.0.3, Python 3.13, macOS arm64

Reproduction

import io, os
from magika import Magika

m = Magika()

rp, wp = os.pipe()
os.write(wp, b"hello world this is a test\n" * 10)
os.close(wp)
buf = io.BufferedReader(io.FileIO(rp, closefd=True))

print(isinstance(buf, io.BufferedIOBase), buf.readable(), buf.seekable())
# True True False   -> passes every guard in identify_stream()

m.identify_stream(buf)

Equivalent one-liner:

echo hi | python3 -c "import sys; from magika import Magika; Magika().identify_stream(sys.stdin.buffer)"

Actual result

True True False
Traceback (most recent call last):
  File ".../magika/magika.py", line 204, in identify_stream
    current_position = stream.tell()
OSError: [Errno 29] Illegal seek

During handling of the above exception, another exception occurred:
...
UnboundLocalError: cannot access local variable 'current_position' where it is not associated with a value

Expected result

The original OSError (unseekable stream) propagates unchanged. Magika's own error should never shadow the underlying I/O failure.

Root cause

python/src/magika/magika.py:203-209:

try:
    current_position = stream.tell()
    result = self._get_result_from_seekable(Seekable(stream))
finally:
    # seek to the previous position even in case of exceptions
    stream.seek(current_position)
return result

current_position is bound inside the try block but referenced unconditionally in finally. When the first statement (stream.tell()) raises, the name was never bound, so finally raises UnboundLocalError and replaces the real exception.

Impact

Callers cannot distinguish "this stream is not seekable" from a Magika internal bug: the traceback names a Magika local variable and the actionable OSError is demoted to __context__. This is the exact path taken by anyone piping data into a Python tool that forwards sys.stdin.buffer to identify_stream().

Proposed fix

Hoist the tell() out of the try, so that finally only runs once the position is known:

current_position = stream.tell()
try:
    result = self._get_result_from_seekable(Seekable(stream))
finally:
    stream.seek(current_position)
return result

The position-restoring guarantee is preserved, because the only statement remaining inside try is the one that needs the restore.

I have a fix plus a regression test in python/tests/test_magika_python_module.py (next to test_magika_module_identify_stream_does_not_alter_position) and will open a PR shortly.