Fix empty filename when Content-Disposition header is missing in /dl command

[Copilot]

Direct link downloads via /dl fail when the server omits Content-Disposition header—filename remains empty, causing saves to invalid paths like /Telegram/ instead of /Telegram/file.mp4.

Changes

• core/tasks/directlinks/util.go: Add filenameFromURL() to extract filename from URL path with URL decoding support
• core/tasks/directlinks/execute.go: Fall back to URL-based filename extraction when Content-Disposition is empty; return error if filename cannot be determined from either source
• core/tasks/directlinks/util_test.go: Unit tests covering encoded filenames, edge cases, and malformed URLs

Example

// Before: filename only set from Content-Disposition
if name := resp.Header.Get("Content-Disposition"); name != "" {
    file.Name = parseFilename(name)
}

// After: fallback to URL path extraction
if name := resp.Header.Get("Content-Disposition"); name != "" {
    file.Name = parseFilename(name)
}
if file.Name == "" {
    file.Name = filenameFromURL(file.URL)  // e.g., "https://example.com/video.mp4" → "video.mp4"
}
if file.Name == "" {
    return fmt.Errorf("could not determine filename for %s", file.URL)
}

Original prompt

---

This section details on the original issue you should resolve

<issue_title>/dl下载直链若Content-Disposition为空，则文件名为空任务失败</issue_title>
<issue_description>### 👾 Description

/dl下载直链若Content-Disposition为空，则文件名为空任务失败

⚡️ Expected Behavior

文件名正常获取

📄 Configuration File

# 存储列表
[[storages]]
# 标识名, 需要唯一
name = "189cloud"
# 存储类型, 目前可用: local, alist, webdav, s3, telegram
type = "webdav"
# 启用存储
enable = true
# 文件保存根路径
url = "http://192.168.2.5:5244/dav" # Alist 的 URL
username = "admin"  # Alist 的用户名
password = "x" # Alist 的密码
base_path = "/189cloud/来自家庭共享/Telegram" # Alist 中的基础路径, 所有文件将存储在此路径下

🔍 Logs

显示传输成功，但没见到文件：INFO <webdav/webdav.go:55> webdav[189cloud]: Saving file to 08:23:27 DEBU <webdav/webdav.go:81> webdav[189cloud]: Checking if file exists at /189cloud/来自家庭共享/Telegram 08:23:31 DEBU <webdav/webdav.go:81> webdav[189cloud]: Checking if file exists at /189cloud/来自家庭共享/Telegram_1 08:23:31 DEBU <webdav/webdav.go:81> webdav[189cloud]: Checking if file exists at /189cloud/来自家庭共享/Telegram_2 08:23:31 DEBU <webdav/webdav.go:81> webdav[189cloud]: Checking if file exists at /189cloud/来自家庭共享/Telegram_3 08:23:55 ERRO <directlinks/execute.go:134> Failed to close and remove cache file: close cache/direct_d5ri8990kahs73accqpg_: file already closed 08:23:55 INFO <directlinks/execute.go:100> Directlinks task d5ri8990kahs73accqpg completed successfully 08:23:55 INFO <directlinks/progress.go:78> Parsed task d5ri8990kahs73accqpg completed successfully 08:23:56 INFO <core/core.go:50> Task d5ri8990kahs73accqpg completed successfully

翻阅日志发现是/dl没有获取到文件名，正常日志：6:19:35 DEBU <tfile/progress.go:81> Progress update: 10月17日(6).mp4, 401331932/401331932 06:19:36 INFO <tfile/execute.go:47> file[10月17日(6).mp4]: File downloaded successfully 06:19:36 INFO <webdav/webdav.go:55> webdav[189cloud]: Saving file to 10月17日(6).mp4 06:19:36 DEBU <webdav/webdav.go:81> webdav[189cloud]: Checking if file exists at /189cloud/来自家庭共享/Telegram/10月17日(6).mp4 06:20:44 DEBU <tfile/progress.go:126> Progress done for file [10月17日(6).mp4] 怎么解决
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes /dl下载直链若Content-Disposition为空，则文件名为空任务失败 #188

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