OAuth PKCE token exchange fails with invalid_grant against strict OAuth 2.1 servers

[gkatz2]

Bug description

Connecting ToolHive to a remote MCP server that uses a strict OAuth 2.1 implementation (such as Datadog's mcp.datadoghq.com) fails during the PKCE token exchange. The user completes the browser-based authorization successfully, but the token exchange silently fails.

When constructing oauth2.Endpoint structs, ToolHive does not set AuthStyle, which defaults to oauth2.AuthStyleAutoDetect (the zero value). Go's golang.org/x/oauth2 library then uses the following auto-detect strategy:

1. First attempt: HTTP Basic Auth — sends Authorization: Basic base64(client_id:) (empty password for public clients)
2. If that fails: Retry with client_id in the POST body

For public PKCE clients registered with token_endpoint_auth_method=none, strict OAuth 2.1 servers reject the Basic Auth attempt — but consume the single-use authorization code in the process. The library's retry with client_id in the POST body then fails with invalid_grant because the code was already burned by the first attempt.

This affects three code paths where oauth2.Endpoint is constructed without AuthStyle:

• pkg/auth/oauth/flow.go — authorization code exchange (the primary failure path)
• pkg/auth/remote/handler.go — token refresh from cached tokens
• pkg/registry/auth/oauth_token_source.go — registry auth token refresh

Steps to reproduce

1. Run a remote MCP server that uses a strict OAuth 2.1 implementation (e.g., Datadog):

   thv run --name datadog-mcp https://mcp.datadoghq.com/api/unstable/mcp-server/mcp
   

2. Complete the browser-based OAuth authorization when prompted
3. Observe the result of the token exchange

Expected behavior

Token exchange succeeds on the first attempt by sending client_id in the POST body, matching the token_endpoint_auth_method=none registration.

Actual behavior

Token exchange fails with:

invalid_grant: authorization code has already been used

The auto-detect logic sends HTTP Basic Auth first, which the strict server rejects while consuming the authorization code. The retry with the correct auth style fails because the code is single-use.

Environment

• ToolHive version: Reproduced on v0.12.1
• OS: macOS (likely all platforms — the issue is in Go library behavior, not OS-specific)