Reject empty metadata keys in MCP validation
HA
1d ago+41+41+01234567890123456789-1-1-0123456789
Hold unprefixed MCP metadata keys to having at least one valid character, rejecting empty keys.
Under MCP 2026-07-28, keys without a prefix must begin and end with an alphanumeric character. Previously, metadata_key permitted an empty string when prefix_required was false because name.empty() evaluated to true.
Add regressions across request meta, notification, and result writer validation.