feat(jar): JWT-Secured Authorization Requests (JAR) support

[jd3vi1]

Summary

Adds support for JWT-Secured Authorization Requests (JAR, RFC 9101). When requestObjectSigningKey is configured, the SDK signs all authorization parameters into a JWT and sends it as the request parameter, so the /authorize redirect carries only client_id and request.

What's included

• requestObjectSigningKey — private key used to sign the request object. Accepts the same formats as clientAssertionSigningKey (PEM string, Buffer, KeyObject, JWK, CryptoKey).
• requestObjectSigningAlg — required whenever requestObjectSigningKey is set. Web Crypto algorithm names are not valid JWA alg values, so an explicit JWA algorithm is required (a shared ASYMMETRIC_SIGNING_ALGS list is used, matching clientAssertionSigningAlg).
• requestObjectSigningKeyId — optional kid header on the request object JWT.
• Works transparently on top of the existing PAR support. When PAR is enabled, the signed request object is POSTed to /oauth/par, which is the recommended FAPI pattern.
• The request object's aud is set to the issuer identifier advertised in the discovery document (which may differ from issuerBaseURL, e.g. a trailing slash), as JAR requires.

Usage

app.use(
  auth({
    authorizationParams: { response_type: 'code' },
    requestObjectSigningKey: fs.readFileSync('./request-object-key.pem'),
    requestObjectSigningAlg: 'RS256',
    pushedAuthorizationRequests: true, // recommended
  }),
);

Tests

• Unit tests for buildRequestObject (header, kid, standard claims, aud), config validation (alg required with key), and the login redirect (request param present, PAR combination).
• An end-to-end test covering the combined PAR + JAR flow.
• Type tests in index.test-d.ts.
• Documentation in EXAMPLES.md and a runnable example at examples/jar.js.

[jd3vi1]

Addressed the nits in 7ff0457:

1. aud fallback removed. buildRequestObject now requires the discovered issuer as its audience argument and throws if it is missing, so the issuerBaseURL fallback (the mismatch that triggers invalid_request_object) is gone. The unit tests now pass the discovered issuer explicitly and assert aud equals it.
2. Non-PAR redirect coverage added. New test asserts the non-PAR authorize redirect carries only client_id and request.
3. Empty-string key. requestObjectSigningKey now rejects '' at config validation, so the "is set" check is consistent between the schema and the login-side guard (previously '' forced alg required but silently skipped JAR).

Full suite and tsd green.