Fix GitHub brand casing in SDK public surface

Generated identifiers and docs spelled the brand as "Github" instead of
"GitHub". The names derive from schema titles/definition keys and from
lowercase wire values, so the fix is applied in codegen rather than by
hand-editing generated output.

- scripts/codegen/utils.ts: add shared brand normalization (loadSchemaJson)
  that corrects title/description/markdownDescription, \ name segments,
  and definitions/\ keys, with collision detection. Wire values such as
  "github" and "github_reference" are left lowercase.
- Per-language casers fixed: typescript, python, csharp (fixBrandCasing),
  rust (normalizeSchemaBrandCasing + fixBrandCasing), and go via the existing
  identifier override map ("github" -> "GitHub").
- java/scripts/codegen/java.ts: local copies of the same helpers wired into
  toPascalCase/toJavaClassName and the schema load sites.
- Regenerated nodejs, dotnet, python, go, and rust outputs.
- Java attachment union types are no longer emitted by the generator (Object
  fallback for the 5-branch anyOf); hand-corrected the casing and renamed the
  stale UserMessageAttachment*/SendAttachment* GitHub reference files.
- Updated E2E tests referencing the renamed symbols (go, dotnet, python, rust)
  and corrected GitHubToken casing in docs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: stephentoub

.github/agents/docs-maintenance.agent.md

@@ -377,7 +377,7 @@ cat go/types.go | grep -A 15 "type SessionHooks struct"
 ```
 
 **Must match (PascalCase for exported):**
-- `ClientOptions` fields: `CLIPath`, `CLIUrl`, `UseStdio`, `Port`, `LogLevel`, `AutoStart`, `Env`, `GithubToken`, `UseLoggedInUser`
+- `ClientOptions` fields: `CLIPath`, `CLIUrl`, `UseStdio`, `Port`, `LogLevel`, `AutoStart`, `Env`, `GitHubToken`, `UseLoggedInUser`
 - `SessionConfig` fields: `Model`, `Tools`, `Hooks`, `SystemMessage`, `MCPServers`, `AvailableTools`, `ExcludedTools`, `Streaming`, `ReasoningEffort`, `Provider`, `InfiniteSessions`, `CustomAgents`, `WorkingDirectory`
 - `Session` methods: `Send()`, `SendAndWait()`, `GetMessages()`, `Disconnect()`, `Abort()`, `ExportSession()`
 - Hook fields: `OnPreToolUse`, `OnPostToolUse`, `OnPostToolUseFailure`, `OnUserPromptSubmitted`, `OnSessionStart`, `OnSessionEnd`, `OnErrorOccurred`
@@ -395,7 +395,7 @@ cat dotnet/src/Types.cs | grep -A 15 "public class SessionHooks"
 ```
 
 **Must match (PascalCase):**
-- `CopilotClientOptions` properties: `CliPath`, `CliUrl`, `UseStdio`, `Port`, `LogLevel`, `AutoStart`, `Environment`, `GithubToken`, `UseLoggedInUser`
+- `CopilotClientOptions` properties: `CliPath`, `CliUrl`, `UseStdio`, `Port`, `LogLevel`, `AutoStart`, `Environment`, `GitHubToken`, `UseLoggedInUser`
 - `SessionConfig` properties: `Model`, `Tools`, `Hooks`, `SystemMessage`, `McpServers`, `AvailableTools`, `ExcludedTools`, `Streaming`, `ReasoningEffort`, `Provider`, `InfiniteSessions`, `CustomAgents`, `WorkingDirectory`
 - `CopilotSession` methods: `SendAsync()`, `SendAndWaitAsync()`, `GetMessagesAsync()`, `DisposeAsync()`, `AbortAsync()`, `ExportSessionAsync()`
 - Hook properties: `OnPreToolUse`, `OnPostToolUse`, `OnPostToolUseFailure`, `OnUserPromptSubmitted`, `OnSessionStart`, `OnSessionEnd`, `OnErrorOccurred`

docs/auth/authenticate.md

@@ -167,7 +167,7 @@ func main() {
 import copilot "github.com/github/copilot-sdk/go"
 
 client := copilot.NewClient(&copilot.ClientOptions{
-    GithubToken:     userAccessToken,   // Token from OAuth flow
+    GitHubToken:     userAccessToken,   // Token from OAuth flow
     UseLoggedInUser: copilot.Bool(false), // Don't use stored CLI credentials
 })
 ```

docs/setup/github-oauth.md

@@ -206,7 +206,7 @@ func main() {
 ```go
 func createClientForUser(userToken string) *copilot.Client {
     return copilot.NewClient(&copilot.ClientOptions{
-        GithubToken:     userToken,
+        GitHubToken:     userToken,
         UseLoggedInUser: copilot.Bool(false),
     })
 }

docs/troubleshooting/debugging.md

@@ -292,7 +292,7 @@ var client = new CopilotClient(new CopilotClientOptions
 
    ```go
    client := copilot.NewClient(&copilot.ClientOptions{
-       GithubToken: os.Getenv("GITHUB_TOKEN"),
+       GitHubToken: os.Getenv("GITHUB_TOKEN"),
    })
    ```
    </details>
@@ -303,7 +303,7 @@ var client = new CopilotClient(new CopilotClientOptions
    ```csharp
    var client = new CopilotClient(new CopilotClientOptions
    {
-       GithubToken = Environment.GetEnvironmentVariable("GITHUB_TOKEN")
+       GitHubToken = Environment.GetEnvironmentVariable("GITHUB_TOKEN")
    });
    ```
    </details>