all: stabilize client OAuth support (#861)

Also fix lint errors that were hidden because of the build tag.

internal/docs/protocol.src.md

@@ -232,11 +232,6 @@ The  [_auth middleware example_](https://github.com/modelcontextprotocol/go-sdk/
 
 ### Client
 
-> [!IMPORTANT]
-> Client-side OAuth support is currently experimental and requires the `mcp_go_client_oauth` build tag to compile.
-> API changes may still be made, based on developer feedback. The build tag will be removed in `v1.5.0`, which
-> is planned to be released by the end of March 2026.
-
 Client-side authorization is supported via the
 [`StreamableClientTransport.OAuthHandler`](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk/mcp#StreamableClientTransport.OAuthHandler)
 field. If the handler is provided, the transport will automatically use it to

internal/oauthtest/fake_authorization_server.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by the license
 // that can be found in the LICENSE file.
 
-//go:build mcp_go_client_oauth
-
 package oauthtest
 
 import (