Fix errcheck violation in RSS feed handler

[Copilot]

The Feed handler in internal/handlers/blog.go had two unchecked w.Write() calls, violating errcheck linting rules.

Changes

• Check error return from w.Write() in the RSS feed handler
• Combine XML header and data into single write operation to prevent partial responses
• Use generic error message instead of exposing err.Error() to clients
• Pre-allocate output buffer with exact capacity to avoid reallocation

Before

w.Write([]byte(xml.Header))
w.Write(data)

After

output := make([]byte, 0, len(xml.Header)+len(data))
output = append(output, []byte(xml.Header)...)
output = append(output, data...)
if _, err := w.Write(output); err != nil {
    // Cannot send error response after w.Write has been called
    return
}

Single write ensures atomicity—either the complete RSS feed is sent or nothing is written to the response body.

Original prompt

Pull Request: #21

Error: internal/handlers/blog.go:168:9: Error return value of w.Write is not checked (errcheck)

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