fix: resolve default values from emitter options JSON schema

[Copilot]

Minimal fix — only 4 files changed, no breaking changes to existing tests.

Changes Made

• schema-validator.ts: Add useDefaults option to JSONSchemaValidatorOptions interface and pass it through to AJV (+2 lines)
• library.ts: Enable useDefaults: true when creating the emitter option validator (+1 line)
• emitter-options.test.ts: Add 3 test cases for schema defaults — applies defaults when no options provided, user values override defaults, and partial defaults for missing options (+76 lines)
• Changelog entry: Documents the fix for @typespec/compiler

How It Works

The fix enables AJV's built-in useDefaults option specifically for the emitter option validator. When an emitter defines default values in its JSON schema options, those defaults are now applied to context.options during $onEmit. This is a non-breaking, additive change — existing emitters without default values in their schemas are unaffected.

Testing

• ✅ All 8 emitter options tests pass (1 pre-existing skip)
• ✅ No changes to any existing tests required

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Bug]: EmitterOptions defaults are not resolved.</issue_title>
<issue_description>### Describe the bug

Following the same example of custom configs definitions from here: https://typespec.io/docs/extending-typespec/emitters-basics/#custom-configuration-options, but using default for some property, doesn't in runtime doesn't result in default value being passed into emitter, so the typing is off.

Reproduction

E.g. having this from documentation:

export interface EmitterOptions {
  "target-name": string;
}

const EmitterOptionsSchema: JSONSchemaType<EmitterOptions> = {
  type: "object",
  additionalProperties: false,
  properties: {
    "target-name": { type: "string", nullable: true, default: null },
  },
  required: [],
};

In emitter

export const $lib = createTypeSpecLibrary({
  internal: internalLib,
  emitter: {
    options: EmitterOptionsSchema,
  },
});

export async function $onEmit(context: EmitContext<EmitterOptions>) {
  console.log(contenxt.options); // --> {}
}

If I'm providing json-schema, I'd expect defaults set in it to be used. Otherwise it doesn't make a lot of sense and usage of json-schema in this case should be documented on the terms of scopes it's being applied for. R.n. seems like only for IDE autocomplete integration for emitters, but no types validation nor defaults substitution.

Checklist

• Follow our Code of Conduct
• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion.
• The provided reproduction is a minimal reproducible example of the bug.</issue_description>

<agent_instructions>Find any issues in the backlog that address this deficiency</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@markcowl @markcowl see if any duplicate issues

• Fixes [Bug]: EmitterOptions defaults are not resolved. #8489

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