Fix silent wget failures in DownloadFile causing corrupted .bak file downloads

[Copilot]

When downloading .bak files from HTTP servers during container creation, users experienced mysterious "corrupted file" errors despite files appearing to download successfully. The issue was caused by silent wget failures that weren't being detected or reported.

Problem

Users running commands like:

sqlcmd create mssql --name express --accept-eula --using http://localhost:3000/files/WideWorldImporters-Full.bak

Would see:

• Files appeared to download (correct size shown in container)
• But database restore failed with "The volume on device '/var/opt/mssql/backup/WideWorldImporters-Full.bak' is empty"
• No error messages during the download process

Root Cause

The DownloadFile function in internal/container/controller.go was:

1. Ignoring wget errors: The function called runCmdInContainer() but ignored the returned stdout/stderr
2. Using incompatible wget flags: GNU wget flags didn't work with BusyBox wget in Alpine containers
3. Not validating downloads: No verification that files actually existed after download

When containers tried to access localhost URLs (which fails due to container networking), wget would fail silently.

Solution

1. Added Comprehensive Error Detection

// Before: Silent failures
c.runCmdInContainer(id, cmd)

// After: Proper error checking
stdout, stderr = c.runCmdInContainer(id, cmd)
if len(stderr) > 0 {
    stderrStr := string(stderr)
    if strings.Contains(stderrStr, "Connection refused") ||
       strings.Contains(stderrStr, "404 Not Found") ||
       // ... other error conditions {
        panic("wget download failed: " + stderrStr)
    }
}

2. Fixed BusyBox Compatibility

// Before: GNU wget flags (unsupported)
cmd = []string{"wget", "--no-check-certificate", "--timeout=300", "--tries=3", "--progress=dot:mega", "-O", file, src}

// After: BusyBox-compatible flags
cmd = []string{"wget", "-T", "300", "-t", "3", "-O", file, src}

3. Added File Validation

• Verify downloaded files actually exist
• Provide clear error messages when downloads fail

User Impact

Before Fix

Downloading WideWorldImporters-Full.bak
Restoring database WideWorldImporters-Full
ERROR: The volume on device '/var/opt/mssql/backup/WideWorldImporters-Full.bak' is empty.

After Fix

Downloading WideWorldImporters-Full.bak
panic: wget download failed: Connecting to 127.0.0.1:3000 (127.0.0.1:3000)
wget: can't connect to remote host (127.0.0.1): Connection refused

Users now get clear, actionable error messages and can use proper solutions:

• External URLs: https://example.com/files/database.bak
• Host networking: http://host.docker.internal:3000/files/database.bak
• Proper container networking configuration

Testing

• Added comprehensive error detection tests
• Verified existing functionality remains intact
• Added documentation about container networking limitations

Fixes #566.

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