discoverMetadataWithFallback (auth)

Performs OAuth authorization/protected-resource metadata discovery, trying path-aware well-known URL first and falling back to root if needed. Centralizes resilient discovery for auth flows.

async function discoverMetadataWithFallback(
    serverUrl: string | URL,
    wellKnownType: 'oauth-authorization-server' | 'oauth-protected-resource',
    fetchFn: FetchLike,
    opts?: { protocolVersion?: string; metadataUrl?: string | URL; metadataServerUrl?: string | URL }
): Promise<Response | undefined> {
    const issuer = new URL(serverUrl);
    const protocolVersion = opts?.protocolVersion ?? LATEST_PROTOCOL_VERSION;

    let url: URL;
    if (opts?.metadataUrl) {
        url = new URL(opts.metadataUrl);
    } else {
        // Try path-aware discovery first
        const wellKnownPath = buildWellKnownPath(wellKnownType, issuer.pathname);
        url = new URL(wellKnownPath, opts?.metadataServerUrl ?? issuer);
        url.search = issuer.search;
    }

    let response = await try

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