fix(packaging): expose Ajv from CJS validator subpaths

[morluto]

Summary

Fixes the CommonJS validators/ajv subpath export for both @modelcontextprotocol/server and @modelcontextprotocol/client. In CommonJS consumers, reading Ajv from either AJV validator subpath currently throws ReferenceError: import_ajv is not defined.

The CJS build was preserving Ajv as a live getter but leaving it pointed at an ESM-only local alias (import_ajv). Exporting Ajv as a concrete local value from the provider makes the generated CJS subpath re-export require_ajvProvider.Ajv, matching the actual CJS provider module.

Adds regression coverage through the existing dist/barrel tests by requiring the built CJS validator subpath and constructing Ajv, addFormats, and AjvJsonSchemaValidator.

Fixes #2430.

Verification

• pnpm --filter @modelcontextprotocol/server --filter @modelcontextprotocol/client build
• node -e "const m = require('./packages/server/dist/validators/ajv.cjs'); new m.Ajv({ strict: false })"
• node -e "const m = require('./packages/client/dist/validators/ajv.cjs'); new m.Ajv({ strict: false })"
• pnpm --filter @modelcontextprotocol/server test -- test/server/barrelClean.test.ts
• pnpm --filter @modelcontextprotocol/client test -- test/client/barrelClean.test.ts
• pnpm --filter @modelcontextprotocol/core-internal test -- test/validators/validators.test.ts
• node scripts/smoke-dist-types.mjs
• git diff --check

[changeset-bot]

🦋 Changeset detected

Latest commit: c269c39

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

This PR includes changesets to release 2 packages

Name	Type
@modelcontextprotocol/server	Patch
@modelcontextprotocol/client	Patch

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commit: c269c39

[koriyoshi2041]

Verified this locally against the reported CJS failure path.

On current origin/main, after a focused server/client/core build, requiring both generated CJS validator subpaths still throws when reading Ajv:

./packages/server ReferenceError: import_ajv is not defined
./packages/client ReferenceError: import_ajv is not defined

On this PR head, the same focused build plus CJS probe succeeds for both packages:

./packages/server function function function true
./packages/client function function function true

I also ran the focused package tests that include the new CJS AJV coverage:

pnpm --filter @modelcontextprotocol/server test -- test/server/barrelClean.test.ts
# 36 passed / 403 tests passed

pnpm --filter @modelcontextprotocol/client test -- test/client/barrelClean.test.ts
# 26 passed / 699 tests passed

So this matches the published/fresh-main reproduction from #2430 and fixes the runtime access path for both server and client.