MCP/schema: validation hardening + anchor-form $ref retention (PR #48638 series)

Context

Part of the fork hardening series proposed in #48638 (branch session-diff-events). Tracker: #49565.

Problem

MCP tool-schema handling: prototype-pollution-prone maps, missing dialect/URN/anchor validation, duplicate anchors, hostile-shape compilation, dangling $ref pruning that also dropped resolvable anchor-form refs, and a hotfix for invalid recursive-schema placeholders (required:{} / empty combinators) that could brick tool schemas.

Change set (commit list, incl. catalog halves of mixed commits)

• 6db8ab6748 mcp schema hotfix
• 02750b7401 wave E schema
• b1f1a08bf2 wave C schema
• 0edfce0205 wave J-C catalog anchor+regex
• 2b95854ed3 wave M-C catalog ref
• (catalog halves of 1ee7d234ec / 88777b8591 to be split for clean stacks)

Validation

• Null-prototype maps, dialect/URN/anchor validation, duplicate-anchor removal; anchor-form $ref retained iff resolvable in same resource, dangling pruned; all corpora compile under ajv; catalog suite 32/0; mock capture: 0 required:{}, 0 empty combinators.

Linked: PR #48638 · tracker #49565 · sibling tracking issues listed in a follow-up comment.

Start with the commits listed in the issue and the linked PR #48638, then review tracker #49565 and the follow-up comment for scope. Validate the MCP schema changes against the stated criteria: catalog suite 32/0, all corpora compile under ajv, resolvable anchor-form refs are retained, and mock capture reports no invalid required objects or empty combinators.