fix(storage): fix Azure Blob dump failing with file does not exist (#38814)

## Issue

Gitea fails to dump LFS (and other object-storage) files when Azure Blob
Storage is configured as the storage backend. The dump reports:

Failed to dump LFS objects: /file/path: copying contents: file does not
exist

This happens with any non-empty base path (the default for LFS storage),
which is why the user could only work around it by using `--skip-*`
flags.

The root cause is in `AzureBlobStorage.IterateObjects()`: Azure's list
API already returns each blob's name including the configured base path,
but the code was building the read client by running that name through
the base-path-prepending helper a second time. This doubled the base
path (e.g. `gitea-lfs/gitea-lfs/aa/bb/hash`), pointing at a blob that
doesn't exist. `Stat()` still succeeded because it doesn't touch the
network, so the failure only surfaced when the dumper actually tried to
read the object's contents.

## Solution

Add `getBlobClientByFullName()`, which builds a blob client from a name
that is already fully qualified, without re-applying
`buildAzureBlobPath()`. `IterateObjects()` now uses it for names
obtained from Azure's list API. `getBlobClient()` (used by `Open`,
`Stat`, `Delete`, `ServeDirectURL`, which take relative paths) is
unchanged in behavior.

Also add `TestAzureBlobStorageDumpArchive`, a regression test that
drives the real dump path (`IterateObjects` → `Stat` →
`dump.Dumper.AddFileByReader` → `mholt/archives` zip writer) against a
**non-empty** `BasePath`, and verifies the produced archive contains the
object with the correct content. The existing Azure tests use an empty
`BasePath` and never read object content via `IterateObjects`, which is
why they didn't catch this.

Fixes #35476

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/storage/storage.go

@@ -11,11 +11,13 @@ import (
 	"net/url"
 	"os"
 	"path"
+	"strings"
 
 	"gitea.dev/modules/httplib"
 	"gitea.dev/modules/log"
 	"gitea.dev/modules/public"
 	"gitea.dev/modules/setting"
+	"gitea.dev/modules/util"
 )
 
 // ErrURLNotSupported represents url is not supported
@@ -139,6 +141,23 @@ func SaveFrom(objStorage ObjectStorage, path string, callback func(w io.Writer)
 	return err
 }
 
+func buildObjectStorePath(base, p string) string {
+	p = strings.TrimPrefix(util.PathJoinRelX(base, p), "/") // object store doesn't use slash for root path
+	if p == "." {
+		p = "" // object store doesn't use dot as relative path
+	}
+	return p
+}
+
+func buildObjectStorePathPrefix(base, p string) string {
+	// ending slash is required for avoiding matching like "foo/" and "foobar/" with prefix "foo"
+	p = buildObjectStorePath(base, p) + "/"
+	if p == "/" {
+		p = "" // object store doesn't use slash for root path
+	}
+	return p
+}
+
 var (
 	// Attachments represents attachments storage
 	Attachments ObjectStorage = uninitializedStorage