fix metadata logic

[n0vad3v]

This PR fixes a recursion issue in metadata handling that could cause requests to hang when the metadata directory is not writable (for example, due to disk full or permission errors). ReadMetadata and WriteMetadata now return explicit errors, recursive self-calls were removed, and call sites were updated to handle failures gracefully. Tests were added to cover both normal metadata flow and failure cases for unavailable metadata paths.

Aims to solve: #453

[github-actions]

Report Summary

┌──────────────────────────────────────────────┬──────────┬─────────────────┬─────────┐
│                    Target                    │   Type   │ Vulnerabilities │ Secrets │
├──────────────────────────────────────────────┼──────────┼─────────────────┼─────────┤
│ ghcr.io/webp-sh/webp_server_go (debian 13.5) │  debian  │        0        │    -    │
├──────────────────────────────────────────────┼──────────┼─────────────────┼─────────┤
│ usr/bin/webp-server                          │ gobinary │        0        │    -    │
└──────────────────────────────────────────────┴──────────┴─────────────────┴─────────┘
Legend:
- '-': Not scanned
- '0': Clean (no security findings detected)