fix: complete SEP-2106 output typing and schema safety

[mattzcarey]

Previously unaddressed

#2337 implemented SEP-2106's runtime and wire behavior, but explicitly left outputSchema disconnected from the tool handler's return type and did not add the SEP's schema-complexity guards.

This PR

• infers successful structuredContent from outputSchema, while preserving error results, input-required results, and schema-less tools
• bounds depth and subschema count in built-in validators and rejects non-local references before compilation
• leaves custom validators and custom Ajv instances in control of their own safety policy

Runtime validation remains authoritative; intentional negative fixtures use an explicit unsafe cast.

Verification

• core-internal: 1,341 tests
• server: 439 tests
• affected e2e scenarios: 490 passed, 26 expected-fail
• pnpm check:all
• pnpm build:all

Follow-up to #2249, #2337, and #2286.

[changeset-bot]

🦋 Changeset detected

Latest commit: ea5c666

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages

Name	Type
@modelcontextprotocol/client	Patch
@modelcontextprotocol/server	Minor
@modelcontextprotocol/express	Major
@modelcontextprotocol/fastify	Major
@modelcontextprotocol/hono	Major
@modelcontextprotocol/node	Major

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commit: ea5c666