Clean up proxy.php by extracting request, status, and streaming helpers

[joshtrichards]

Summary

This branch is a refactor/cleanup of proxy.php with no intended functional change. It improves readability and maintainability by reorganizing the script into focused helper functions, consolidating the main flow, and clarifying comments and names.

• Extracts status handling, request parsing, multipart reconstruction, header forwarding, and response streaming into dedicated helpers.
• Replaces the custom startsWith() helper with native str_starts_with().
• Renames key variables and functions for consistency and readability.
• Clarifies comments and debug messages without intentionally changing proxy behavior.
• Adds type hints where practical.

Notes for reviewers

This is intended to be a structural refactor for readability and maintainability, so review is easiest by comparing control-flow equivalence between the extracted helpers and the prior inline logic. Each extraction is also in its own commit to further ease reviewability.

Tested locally with Nextcloud v33.