fix: handle unsigned 64-bit manifest IDs throughout the pipeline

[jeremybernstein]

Summary

• Follow-up to Fix NumberFormatException for manifest GIDs exceeding Long.MAX_VALUE #8 which fixed the initial NumberFormatException in getSteam3DepotManifest parsing. That fix was incomplete — manifest IDs still rendered as negative signed longs in CDN URLs, file paths, log messages, and binary KeyValue UINT64 deserialization.
• This PR fixes all remaining sites: CDN Client URL construction, Util save/load manifest filenames, DepotDownloader log messages, Steam3Session log messages, and KeyValue.tryReadAsBinary UINT64 → string conversion.
• Adds parameterized tests for CDN URL construction and KeyValue UINT64 roundtripping, plus tests for Util manifest file path handling.

Apologies for the incomplete fix in #8 — should have traced the full pipeline the first time.

Test plan

• ManifestIdUnsignedTest — CDN URL uses unsigned representation (parameterized)
• ManifestIdUnsignedTest — CDN URL without request code
• ManifestIdUnsignedTest — KeyValue binary UINT64 roundtrips
• UtilManifestFilePathTest — save/load manifest filenames use unsigned ID
• Manual: download depot 3701 for app 3700 (NWN:EE) — confirmed working via mavenLocal