feat: add organization scoping to chats (#23827)

Fixes https://github.com/coder/internal/issues/1436

* Adds organization_id to chats with backfill (workspace org → user org membership → default org)
* No support yet for ACLs (follow-up issue)
- Cross-org workspace binding rejected (both in `CreateChatRequest` and in `create_workspace` tool
- Adds `OrganizationAutocomplete` to `AgentCreateForm`
- Docs updated with `organization_id` in chats-api.md

> 🤖 Written by a Coder Agent. Reviewed by many humans and many agents.

---------

Co-authored-by: Mathias Fredriksson <mafredri@gmail.com>

coderd/mcp_test.go

@@ -1276,7 +1276,7 @@ func TestChatWithMCPServerIDs(t *testing.T) {
 
 	ctx := testutil.Context(t, testutil.WaitLong)
 	client := newMCPClient(t)
-	_ = coderdtest.CreateFirstUser(t, client)
+	firstUser := coderdtest.CreateFirstUser(t, client)
 
 	expClient := codersdk.NewExperimentalClient(client)
 
@@ -1288,6 +1288,7 @@ func TestChatWithMCPServerIDs(t *testing.T) {
 
 	// Create a chat referencing the MCP server.
 	chat, err := expClient.CreateChat(ctx, codersdk.CreateChatRequest{
+		OrganizationID: firstUser.OrganizationID,
 		Content: []codersdk.ChatInputPart{
 			{
 				Type: codersdk.ChatInputPartTypeText,