[Schema] SEP-2106: Harden SchemaValidator against external $ref SSRF and composition DoS #358

Implements the security hardening portion of SEP-2106 for the MCP Spec 2026-07-28 release.

Tracked by umbrella #337.

Spec rationale

Once $ref and rich composition land in inputSchema/outputSchema, naive validators are exposed to: SSRF via external $ref URIs, and DoS via pathological compositions / deep nesting / large subschema counts.

PHP SDK changes

• SchemaValidator MUST NOT auto-dereference external $ref URIs (block any non-same-document reference by default).
• Bound: schema depth, subschema count, validation time.
• Configurable limits with sensible defaults (mirror existing session-store DoS limit ergonomics).
• Add regression tests with adversarial schemas.

Related

• Umbrella [2026-07-28] Schema modernization & error code standardization #337
• Vocabulary sub-issue (introduces $ref support that this hardens)