feat: add appName and appUrl options for app identification headers

[robert-j-y]

Description

Adds appName and appUrl as first-class options on createOpenRouter() so users can set app identification headers without manually constructing a headers object.

Before:

const openrouter = createOpenRouter({
  apiKey: 'sk-...',
  headers: {
    'X-OpenRouter-Title': 'My App',
    'HTTP-Referer': 'https://myapp.com',
  },
});

After:

const openrouter = createOpenRouter({
  apiKey: 'sk-...',
  appName: 'My App',
  appUrl: 'https://myapp.com',
});

Updates since last revision

• Fixed deprecated OpenRouter class in facade.ts: The class now wires appName/appUrl into its baseConfig headers, matching the createOpenRouter() behavior. Previously these options were silently ignored through the deprecated path.

Key details for review

• appName maps to X-OpenRouter-Title, appUrl maps to HTTP-Referer — these are the canonical headers per OpenRouter app attribution docs.
• Header precedence: appName/appUrl are spread before options.headers in both createOpenRouter() and the deprecated OpenRouter class, so explicit headers entries take priority. This is covered by a dedicated test for createOpenRouter().
• Falsy values (empty string, undefined) are not set as headers due to the && guard.

Items for human review

• The changeset file references X-Title — Fixed: changeset now correctly references X-OpenRouter-Title.
• The deprecated OpenRouter class in facade.ts silently ignores appName/appUrl — Fixed: facade.ts now stores and wires both options into headers.
• No dedicated unit tests for the deprecated OpenRouter facade class with appName/appUrl — the 5 existing tests cover createOpenRouter() only. The facade wiring uses the same spread pattern, but has no test coverage.
• The Speakeasy-generated @openrouter/sdk still uses the legacy X-Title header name. This PR intentionally uses the current canonical X-OpenRouter-Title per OpenRouter docs — be aware of the difference across SDKs.
• No end-to-end testing against the live OpenRouter API was performed — only unit tests with a mock server. Headers are verified via test server request inspection.

Changes

• src/provider.ts: Added appName and appUrl to OpenRouterProviderSettings interface; wired into getHeaders() as X-OpenRouter-Title and HTTP-Referer respectively.
• src/facade.ts: Wired appName and appUrl into the deprecated OpenRouter class constructor and baseConfig headers, matching createOpenRouter() behavior.
• src/tests/provider-options.test.ts: 5 new tests covering individual headers, both together, custom header override precedence, and absence when not provided.
• .changeset/add-app-name-url.md: Minor changeset for the new feature.

Checklist

• I have run pnpm stylecheck and pnpm typecheck
• I have run pnpm test and all tests pass (248/248)
• I have added tests for my changes (5 new tests)
• I have updated documentation (if applicable) — README update may be desired as a follow-up

Changeset

• I have run pnpm changeset to create a changeset file (minor)

Link to Devin session: https://app.devin.ai/sessions/a4b5cbff5a0448aa89aa45557aa32161
Requested by: @robert-j-y