Add proper partial content response handling

[jollySleeper]

This change fixes HTTP range request handling issues in the piped-proxy, ensuring proper partial content responses and Content-Range headers are returned when clients request specific byte ranges from video streams. This will fix playback issues on LibreTube (libre-tube/LibreTube#7463)

Problem Solved

The proxy was not properly handling HTTP range requests, causing several issues:

1. Incorrect Status Codes: Range requests were returning 200 OK instead of the correct 206 Partial Content status
2. Missing Content-Range Headers: Responses lacked proper Content-Range headers required for partial content
3. Malformed Range Headers: UMP streams had incorrect Content-Range header formatting
4. Range Truncation Issues: YouTube sometimes truncates ranges (start > end), which wasn't handled properly

Changes Made

New Range Request Handling Infrastructure

Added new structs and functions in src/main.rs:

struct RangeRequest {
    start: u64,
    end: u64,
    total_size: u64,
}

• parse_range(): Parses range strings (e.g., "0-1023") into structured RangeRequest objects with proper bounds checking
• handle_range_response_correction(): Main correction logic that detects range requests and converts 200 OK responses to 206 Partial Content
• apply_range_headers(): Applies proper HTTP headers for partial content responses

Core Fixes

1. Range Response Correction: Added call to handle_range_response_correction() after response building to fix status codes and headers
2. Content-Range Header Fix: Improved UMP stream handling with properly formatted Content-Range headers
3. Range Truncation Handling: Special logic for when YouTube truncates ranges (start > end positions)
4. Bounds Validation: Ensures range requests don't exceed content length

Technical Details

• Status Code Conversion: Automatically converts 200 OK to 206 Partial Content for range requests
• Header Formatting: Generates correct Content-Range: bytes start-end/total headers
• Content-Length Calculation: Properly calculates actual content length for partial responses
• Error Handling: Graceful fallback when range parsing fails or headers are malformed

Benefits

• ✅ HTTP Compliance: Proper adherence to HTTP range request specifications
• ✅ Client Compatibility: Better support for video players that rely on range requests
• ✅ Bandwidth Efficiency: Enables proper resumable downloads and seeking
• ✅ UMP Stream Support: Correct partial content handling for YouTube's UMP streams
• ✅ Robust Parsing: Handles edge cases like truncated ranges and malformed inputs

Testing

The changes include comprehensive error handling and validation:

• Invalid range formats are rejected gracefully
• Missing content-length headers are handled
• Range bounds are validated against content size
• YouTube-specific range truncation scenarios are covered

This fix ensures the proxy correctly handles partial content requests, improving compatibility with media players and enabling features like video seeking and resumable downloads.

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LLM's help were taken to solve this issue 😅