fix: resolve GCS S3-compatible API SignatureDoesNotMatch on put_object

[yaojin3616]

Problem

Using Google Cloud Storage (GCS) with the S3-compatible API causes SignatureDoesNotMatch error on put_object:

botocore.exceptions.ClientError: An error occurred (SignatureDoesNotMatch) when calling the PutObject operation: Invalid argument.

Root Cause

GCS S3-compatible API computes the V4 signature body-hash differently when ContentType is not explicitly set. Boto3 signs with an empty content-type, but GCS applies a different default, causing a signature mismatch.

Fix

• write_bytes(): Always set ContentType explicitly (default application/octet-stream) to prevent GCS signature mismatch
• presign_download_url(): Add comment clarifying GCS needs no path rewrite (unlike MinIO)
• .env.example: Add GCS configuration examples with S3_REGION=auto

GCS Configuration

STORAGE_BACKEND=s3
S3_BUCKET=your-gcs-bucket-name
S3_REGION=auto
S3_ENDPOINT_URL=https://storage.googleapis.com
S3_ACCESS_KEY_ID=your-hmac-access-key
S3_SECRET_ACCESS_KEY=your-hmac-secret
S3_PREFIX=agents