test: Add integration tests and fix bugs found by them

[mojatter]

Summary

Depends on #57.

• Add //go:build integtest integration tests for s3, gcs, and azblob backends
• Fix four bugs discovered during integration testing:
  • s3: HeadObject 404 now correctly maps to s2.ErrNotExist (was only handling NoSuchKey, not NotFound)
  • s3: Buffer PutObject body into bytes.Reader so the AWS SDK can seek for payload signing over HTTP endpoints
  • s3api server: Support x-amz-metadata-directive in CopyObject — REPLACE uses request headers, COPY (default) preserves source metadata
  • azblob: Lowercase metadata keys on read (Azure normalizes them to Title-Case)
• Fix s2test metadata keys to avoid hyphens (rejected by Azure)

Running integration tests

# S3 (against AWS)
S2_TEST_S3_BUCKET=my-bucket go test -tags integtest ./s3/...

# S3 (against local s2-server)
S2_TEST_S3_BUCKET=my-bucket S2_TEST_S3_ENDPOINT=http://localhost:9000 \
  S2_TEST_S3_ACCESS_KEY_ID=user S2_TEST_S3_SECRET_ACCESS_KEY=pass \
  go test -tags integtest ./s3/...

# GCS
S2_TEST_GCS_BUCKET=my-bucket go test -tags integtest ./gcs/...

# Azure Blob Storage
S2_TEST_AZBLOB_CONTAINER=my-container \
  S2_TEST_AZBLOB_ACCOUNT_NAME=acct S2_TEST_AZBLOB_ACCOUNT_KEY=key \
  go test -tags integtest ./azblob/...

Test plan

• All unit tests pass
• golangci-lint run ./... reports 0 issues
• Integration tests pass: AWS S3, S2 server (HTTP), GCS, Azure Blob Storage