[mykytanetipa]

Adds the OAuth runtime manager to run interactive browser PKCE and device code sign-in, refresh and revoke tokens, and attach bearer grants to outgoing A2A client calls. Everything stays inside the internal OAuth package in this PR, keeping token lifecycle and prompt handling separate from CLI flag wiring in #130.

Example:

mgr := &oauth.Manager{
    Clients: []oauth.Registration{{Authority: as.URL, ClientID: "my-cli"}},
    Store:   &oauth.FileStore{Dir: storeDir},
}
httpClient := &http.Client{Transport: mgr.Transport(http.DefaultTransport)}
client, _ := a2aclient.NewFromCard(ctx, card,
    a2aclient.WithInterceptors(mgr.Interceptor()),
    a2aclient.WithJSONRPCTransport(httpClient),
)

resp, err := client.SendMessage(ctx, params)
var ire *oauth.InteractionRequiredError
if errors.As(err, &ire) {
    _, err = mgr.Acquire(signInCtx, ire.Requirement, oauth.AcquireOptions{
        Prompter: oauth.Prompter{Out: os.Stderr},
    })
    resp, err = client.SendMessage(retryCtx, params)
}

Hooking OAuth into the A2A client uses both a client interceptor and an HTTP transport wrapper because neither sees the full call on its own. The interceptor sees the resolved agent card before the first RPC and attaches a cached or freshly refreshed grant up front. On the wire, the HTTP transport catches raw 401 responses and retries once after refreshing or discovering the server. Only the transport can read the challenge header because the SDK drops HTTP headers on non-200 errors. Requests that already carry an Authorization header are left alone, and tokens are only sent over HTTPS or to loopback hosts.

Only a card that requires OAuth makes a missing grant an error. For any other agent, a stored grant is attached when one covers it (the same resource or a parent path, with at least the scopes it asks for), and the request goes out either way, so agents that accept API keys or no credentials are unaffected.

Neither hook ever blocks on a browser or device prompt inside an RPC, since waiting on a human would burn the per-request timeout. When silent auth cannot satisfy the agent, both return an interaction-required error carrying the discovered requirement. The caller can then run sign-in under its own deadline and retry cleanly. Every authorization and token call also passes the target agent URL as the resource indicator. Token requests therefore use standard net/http rather than x/oauth2, whose refresh path drops the resource parameter.

Refs #98