fix: preserve elicit string pattern constraints

[Genmin]

Summary

• Preserve pattern on elicitation string property schemas instead of stripping it during validation.
• Cover both direct StringSchemaSchema parsing and nested ElicitRequestFormParamsSchema parsing.
• Document the temporary generated-spec type mismatch: the written 2025-11-25 elicitation spec includes StringSchema.pattern, while spec.types.ts is still missing it.

Notes

This intentionally keeps the change narrow to the spec-backed string pattern field rather than making all primitive schemas permissive. It addresses the concrete pattern loss described in #1844 without broad passthrough behavior.

Spec reference: https://modelcontextprotocol.io/specification/2025-11-25/client/elicitation

Validation

• pnpm --filter @modelcontextprotocol/core exec vitest run test/types.test.ts test/spec.types.test.ts
• pnpm --filter @modelcontextprotocol/core run typecheck
• pnpm --filter @modelcontextprotocol/core exec prettier --ignore-path ../../.prettierignore --check src/types/schemas.ts test/types.test.ts test/spec.types.test.ts
• pre-push hook: pnpm -r typecheck, pnpm -r build, pnpm sync:snippets --check && pnpm -r lint

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🦋 Changeset detected

Latest commit: 34e7438

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

This PR includes changesets to release 1 package

Name	Type
@modelcontextprotocol/core	Patch

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commit: 34e7438