OIDC upstream clients cannot use client_secret_basic (AuthStyle hardcoded)

Bug description

Confidential OIDC upstream clients have no way to authenticate at the token endpoint with client_secret_basic. Unlike the pure-OAuth2 path (fixed in #6536), this isn't a lossy config translation — it's an unconditional literal in the provider constructor itself.

In pkg/authserver/upstream/oidc.go, NewOIDCProvider builds its oauth2.Config with:

// Create the oauth2.Config for use with golang.org/x/oauth2 library
// Use go-oidc's endpoint which handles discovery, but explicitly set AuthStyle
// to ensure client credentials are sent in the request body (not Basic auth header)
// for consistent behavior across different IDP implementations.
providerEndpoint := oidcProvider.Endpoint()
p.oauth2Config = &oauth2.Config{
    ...
    Endpoint: oauth2.Endpoint{
        AuthURL:   providerEndpoint.AuthURL,
        TokenURL:  providerEndpoint.TokenURL,
        AuthStyle: oauth2.AuthStyleInParams,
    },
}

upstream.OIDCConfig has no TokenEndpointAuthMethod field, and neither does OIDCUpstreamRunConfig (pkg/authserver/config.go) or buildOIDCConfig (pkg/authserver/runner/embeddedauthserver.go). There is no config surface to override this, even in principle.

Why this is a separate issue from #6536

#6536's buildPureOAuth2Config fix added a field and default-filled it before handing off to the existing authStyleFromMethod helper — a translation-layer fix. Here there's no translation layer to fix: the literal lives inside NewOIDCProvider itself, with a comment asserting the choice is deliberate. Closing this gap means adding the field and replacing the hardcoded literal with a call to the same authStyleFromMethod helper the OAuth2 path already uses.

Expected behavior

A confidential OIDC upstream client with a configured secret should default to client_secret_basic (matching RFC 7591 §2's default, and OIDC Discovery 1.0 §3's default when a provider's discovery document omits token_endpoint_auth_methods_supported), while still allowing an explicit override to client_secret_post for providers that require it.

Proposed fix shape

• Add TokenEndpointAuthMethod to CommonOAuthConfig (which upstream.OIDCConfig already embeds), rather than duplicating a parallel field onto OIDCConfig directly — keeps one field/one mapping function shared by both providers instead of two copies that can drift.
• Add the mirrored field to OIDCUpstreamRunConfig; propagate it in buildOIDCConfig the same way buildPureOAuth2Config does today.
• Replace the hardcoded AuthStyle: oauth2.AuthStyleInParams in NewOIDCProvider with authStyleFromMethod(config.TokenEndpointAuthMethod), keeping AuthURL/TokenURL from oidcProvider.Endpoint() (go-oidc's discovery) but letting the configured/defaulted method — not go-oidc's own endpoint guess — decide AuthStyle.

Additional context

• OIDC discovery uniquely publishes token_endpoint_auth_methods_supported, which bare OAuth2 endpoints don't have. A stretch goal beyond "default to Basic" would be respecting that list when present rather than a static default — go-oidc's Provider.Endpoint() doesn't currently expose it, so this would need to read the raw discovery claims separately. Filing this issue scoped to the static default first; the discovery-aware version can be a follow-up if wanted.
• No known-broken real-world IdP identified that would regress from this default, but flagging as an open risk given some non-conformant providers have historically rejected Basic auth.

Start with NewOIDCProvider in pkg/authserver/upstream/oidc.go, then compare the existing field propagation in buildPureOAuth2Config and authStyleFromMethod. Update the shared OIDC configuration path through pkg/authserver/config.go and pkg/authserver/runner/embeddedauthserver.go. Done means OIDC defaults to client_secret_basic while allowing an explicit client_secret_post override.