Add generic WebDAV metadata cache

[taglia]

Summary

This adds a short-lived generic metadata cache for WebDAV item lookups and folder listings.

The cache is used by WebDAV handlers for repeated PROPFIND, GET, HEAD, PUT, DELETE, MOVE, and MKCOL lookup paths, and is invalidated on write operations.

Why

WebDAV clients often issue repeated metadata requests for the same paths during backup and restore workflows. Without a short-lived cache, these requests repeatedly hit Drive metadata APIs even when the state has just been fetched or mutated by the same WebDAV process.

Behavior

• Caches Drive file/folder metadata and folder contents briefly.
• Invalidates affected paths and parent folder listings after mutations.
• Keeps existing WebDAV semantics unchanged.
• Does not add client-specific path matching or backup-system-specific behavior.

Tests

• Updated WebDAV tests to clear cache state between tests.
• Verified WebDAV handler coverage for lookup and mutation paths.
• Verified targeted auth/WebDAV tests locally.