Refactor dav: Adopt two-layer architecture to match other storage providers (#98)

* refactor(dav): Adopt two-layer architecture to match other storage providers

Restructure the DAV client to align with S3, Azure, GCS, and AliOSS provider
patterns in storage-cli. This is the first step in modernizing the DAV
implementation, with advanced features to follow in subsequent PRs.

**Structural Changes:**
- Split client into two layers:
- `client.go`: High-level DavBlobstore implementing storage.Storager
- `storage_client.go`: Low-level StorageClient for HTTP/WebDAV operations
- Remove cmd/ and app/ layers (now integrated directly via storage/factory.go)
- Remove test_assets/ directory (replaced by Docker-based testing)
- Remove old fakes/fake_client.go (replaced by clientfakes/fake_storage_client.go)
- Add helpers.go for shared utilities (getCertPool)

**Stub Methods (to be implemented in follow-up PRs):**
- List: List blobs with optional prefix filter
- Copy: Server-side blob copying via WebDAV COPY
- DeleteRecursive: Delete all blobs matching prefix
- Properties: Retrieve blob metadata
- EnsureStorageExists: Initialize WebDAV directory structure

**Path Handling:**
- Remove automatic 2-character path fan-out. Previously "abcdef..." was stored
as "ab/cd/abcdef...". Now object IDs are passed through unchanged.
- This aligns with other storage-cli providers (S3/Azure/GCS/AliOSS) and
matches existing CCNG usage (BaseClient#partitioned_key already handles
partitioning before calling storage-cli).
- Callers are responsible for any path layout or partitioning strategy.

**Testing:**
- Update tests to use mocked StorageClient with counterfeiter
- Add comprehensive test coverage for Exists (found, not found, error cases)
- Add docker-compose.yml for local WebDAV testing
- Update README with breaking change notice and usage examples

**Code Quality:**
- Align logging style with other providers (lowercase, Debug for success messages)
- All linter checks pass (golangci-lint, go vet, gofmt)
- 100% test pass rate

**Breaking Changes:**
- Removed automatic path partitioning. Object IDs are used exactly as provided.
- Callers must include any desired directory structure in the object ID.
- See dav/README.md "Path Handling Contract" section for migration guidance.

* Update readme

* change log level

Author: kathap

dav/README.md

@@ -6,28 +6,78 @@ WebDAV client implementation for the unified storage-cli tool. This module provi
 
 For general usage and build instructions, see the [main README](../README.md).
 
+## Path Handling Contract
+
+**IMPORTANT:** Callers are responsible for any path layout or partitioning strategy. The DAV driver does not transform object IDs.
+
+Object IDs are used exactly as provided:
+- Simple paths: `my-blob-id` → stored as `my-blob-id`
+- Nested paths: `ab/cd/my-blob-id` → stored as `ab/cd/my-blob-id`
+
+**Breaking change in v0.0.8+:** The driver no longer applies automatic 2-character path fan-out (e.g., `abcdef...` was previously stored as `ab/cd/abcdef...` in v0.0.7 and earlier). Callers must now include any desired directory structure in the object ID itself.
+
+This behavior aligns with other storage-cli providers (S3, Azure, GCS, AliOSS), which also pass keys through unchanged.
+
 ## DAV-Specific Configuration
 
-The DAV client requires a JSON configuration file with WebDAV endpoint details and credentials.
+The DAV client requires a JSON configuration file with the following structure:
+
+``` json
+{
+  "endpoint":        "<string> (required - WebDAV server URL)",
+  "user":            "<string> (optional - for Basic Auth)",
+  "password":        "<string> (optional - for Basic Auth)",
+  "retry_attempts":  <uint> (optional - default: 3),
+  "tls": {
+    "cert": {
+      "ca":          "<string> (optional - PEM-encoded CA certificate)"
+    }
+  },
+  "secret":          "<string> (optional - required for pre-signed URLs)"
+}
+```
 
 **Usage examples:**
 ```bash
-# Upload an object
-storage-cli -s dav -c dav-config.json put local-file.txt remote-object
+# Upload a file to WebDAV
+storage-cli -s dav -c dav-config.json put local-file.txt remote-blob
 
-# Fetch an object
-storage-cli -s dav -c dav-config.json get remote-object local-file.txt
+# Download a file from WebDAV
+storage-cli -s dav -c dav-config.json get remote-blob local-file.txt
 
-# Delete an object
-storage-cli -s dav -c dav-config.json delete remote-object
+# Check if blob exists
+storage-cli -s dav -c dav-config.json exists remote-blob
 
-# Check if an object exists
-storage-cli -s dav -c dav-config.json exists remote-object
+# Delete a blob
+storage-cli -s dav -c dav-config.json delete remote-blob
 
-# Generate a signed URL (e.g., GET for 1 hour)
-storage-cli -s dav -c dav-config.json sign remote-object get 60s
+# Generate a pre-signed URL (requires secret in config)
+storage-cli -s dav -c dav-config.json sign remote-blob get 1h
 ```
 
+## Features
+
+### Basic Operations (Available)
+- **Put** - Upload files to WebDAV server
+- **Get** - Download files from WebDAV server
+- **Delete** - Delete individual blobs
+- **Exists** - Check if a blob exists
+- **Sign** - Generate pre-signed URLs with HMAC-SHA256
+
+### Advanced Operations (Coming Soon)
+The following operations will be available in future releases:
+- **List** - List all blobs or filter by prefix
+- **Copy** - Server-side blob copying via WebDAV COPY method
+- **DeleteRecursive** - Delete all blobs matching a prefix
+- **Properties** - Retrieve blob metadata (ContentLength, ETag, LastModified)
+- **EnsureStorageExists** - Initialize WebDAV directory structure
+
+### Automatic Retry Logic
+All operations automatically retry on transient errors. Default is 3 retry attempts, configurable via `retry_attempts` in config.
+
+### TLS/HTTPS Support
+Supports HTTPS connections with custom CA certificates for internal or self-signed certificates.
+
 ## Pre-signed URLs
 
 The `sign` command generates a pre-signed URL for a specific object, action, and duration.
@@ -44,6 +94,13 @@ The generated URL format:
 
 ### Unit Tests
 Run unit tests from the repository root:
+
 ```bash
-ginkgo --cover -v -r ./dav/...
+go test ./dav/client/...
 ```
+
+Or using ginkgo:
+```bash
+ginkgo --cover -v -r ./dav/client
+```
+