Cache unchanged extension metadata

[Minecraft0122]

Summary

• fingerprint provider and table schema metadata so repeated data gathering only writes metadata when it changes
• keep dynamic table rows out of the fingerprint so row changes still write result data without rewriting table definitions
• key the cache by provider kind, server UUID, plugin, and provider name; no Database instance is retained
• only inspect/store icons for actual table columns, so the fingerprint code does not need a null-icon branch
• invalidate optimistic cache entries when the metadata transaction fails or is skipped under heavy load

Review feedback addressed

• Database is not stored in ExtensionMetadataStorage; DataValueGatherer owns the cache and is recreated when the database changes
• the icon fingerprint helper has no null branch; unused null table slots are not traversed
• transaction retry / MySQL 1205 behavior from the closed PR is not included in this branch

Tests

• unchanged and changed provider metadata
• independent metadata for different server UUIDs
• dynamic table rows versus changed table columns
• skipped and failed metadata transaction invalidation
• transaction successful-versus-actually-executed state
• targeted tests and :common:checkstyleMain / :common:checkstyleTest pass locally

This is the extension-metadata portion of the closed #4772, split into a single-purpose PR as requested.

Closes #2755