storage: support local filesystem storage with localPath config

[huanghaoyuanhhy]

Fixes #964

Summary

Support Milvus Standalone local storage (storageType: local) by adding localPath config that maps to Milvus localStorage.path. This enables backup/restore for Milvus deployed via the official standalone docker method which uses COMMON_STORAGETYPE=local.

Changes

• Add LocalPath field to storage.Config struct
• Add minio.localPath / minio.backupLocalPath config fields with env var support (MINIO_LOCAL_PATH / MINIO_BACKUP_LOCAL_PATH), backupLocalPath inherits from localPath if not set
• Rewrite LocalClient with baseDir (localPath/bucket), absPath(key) and relPath(abs) to correctly resolve relative keys
• BucketExist now checks if baseDir directory exists; CreateBucket creates it via os.MkdirAll
• Update all LocalClient tests to use Config{LocalPath: tempDir, Bucket: "test-bucket"} pattern
• Document new fields in configs/backup.yaml
• Add configs/backup-local.yaml example config for Milvus Standalone local storage
• Add e2e CI test using docker run with embedded etcd, matching official deployment

Breaking Changes

LocalClient now resolves all keys relative to localPath/bucket instead of treating them as absolute paths. Users who previously relied on absolute-path keys with storageType: local must set localPath to the correct base directory (the host path mapping to Milvus localStorage.path).

User Config Example

minio:
  storageType: "local"
  localPath: "volumes/milvus/data"      # host path to Milvus localStorage.path
  bucketName: "a-bucket"
  rootPath: "files"

  backupStorageType: "local"
  backupLocalPath: "volumes/backup"
  backupBucketName: "milvus-backup"
  backupRootPath: "backup"

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This pull-request has been approved by: huanghaoyuanhhy

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.90476% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 28.46%. Comparing base (adae7f7) to head (7cbb895).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/storage/local.go	86.95%	3 Missing and 3 partials ⚠️
core/restore/coll_task.go	76.92%	3 Missing ⚠️
internal/storage/factory.go	0.00%	2 Missing ⚠️

❌ Your project status has failed because the head coverage (28.46%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #985      +/-   ##
==========================================
+ Coverage   28.23%   28.46%   +0.23%     
==========================================
  Files         104      104              
  Lines        9835     9871      +36     
==========================================
+ Hits         2777     2810      +33     
- Misses       6856     6858       +2     
- Partials      202      203       +1     

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