Prompt Templates with Validation

Production apps don't paste strings together. They use prompt templates with named slots, and fail fast when a variable is missing.

Implement render(template, variables):

• Replace each {name} with str(variables[name]). Names are letters, digits and underscores.
• {{ and }} produce literal { and }, so you can include JSON examples in prompts.
• If any placeholder has no value, raise KeyError listing all the missing names, sorted, in the message.
• Extra variables that aren't used are fine.

Also implement placeholders(template) returning the sorted list of distinct names used.

Write it yourself; don't call str.format.