fix: ignore Expect header in S3 REST signer payload

[GayathriSrividya]

closes #3291

Summary

• Strip the Expect header from S3 requests before calling the REST signing service.
• Prevent Expect: 100-continue from being included in the signing payload and signed headers.
• Add a regression test that verifies Expect is not forwarded to the signer service.

Root cause

The S3 REST signer forwarded all request headers to the signer endpoint. Some S3-compatible signing services reject signing payloads that include Expect: 100-continue, resulting in SignError with HTTP 412.

Changes

• In pyiceberg/io/fsspec.py:
  • Remove any Expect header before constructing signer request payload in S3V4RestSigner.call.
• In tests/io/test_fsspec.py:
  • Add test_s3v4_rest_signer_strips_expect_header to validate Expect is removed from both the signer payload and request headers.

Validation

• /opt/homebrew/bin/ruff format pyiceberg/io/fsspec.py tests/io/test_fsspec.py
• /opt/homebrew/bin/ruff check --fix pyiceberg/io/fsspec.py tests/io/test_fsspec.py
• /path/.venv/bin/python -m pytest tests/io/test_fsspec.py -k 's3v4_rest_signer' -xvs