feat: add MCP server configuration backend for chats (#23227)

## Summary

Adds the database schema, API endpoints, SDK types, and encryption
wrappers for admin-managed MCP (Model Context Protocol) server
configurations that chatd can consume. This is the backend foundation
for allowing external MCP tools (Sentry, Linear, GitHub, etc.) to be
used during AI chat sessions.

## Database

Two new tables:
- **`mcp_server_configs`**: Admin-managed server definitions with URL,
transport (Streamable HTTP / SSE), auth config (none / OAuth2 / API key
/ custom headers), tool allow/deny lists, and an availability policy
(`force_on` / `default_on` / `default_off`). Includes CHECK constraints
on transport, auth_type, and availability values.
- **`mcp_server_user_tokens`**: Per-user OAuth2 tokens for servers
requiring individual authentication. Cascades on user/config deletion.

New column on `chats` table:
- **`mcp_server_ids UUID[]`**: Per-chat MCP server selection, following
the same pattern as `model_config_id` — passed at chat creation,
changeable per-message with nil-means-no-change semantics.

## API Endpoints

All routes are under `/api/experimental/mcp/servers/` and gated behind
the `agents` experiment.

**Admin endpoints** (`ResourceDeploymentConfig` auth):
- `POST /` — Create MCP server config
- `PATCH /{id}` — Update MCP server config (full-replace)
- `DELETE /{id}` — Delete MCP server config

**Authenticated endpoints** (all users, enabled servers only for
non-admins):
- `GET /` — List configs (admins see all, members see enabled-only with
admin fields redacted)
- `GET /{id}` — Get config by ID (with `auth_connected` populated
per-user)

**OAuth2 per-user auth flow:**
- `GET /{id}/oauth2/connect` — Initiate OAuth2 flow (state cookie CSRF
protection)
- `GET /{id}/oauth2/callback` — Handle OAuth2 callback, store tokens
- `DELETE /{id}/oauth2/disconnect` — Remove stored OAuth2 tokens

## Security

- **Secrets never returned**: `OAuth2ClientSecret`, `APIKeyValue`, and
`CustomHeaders` are never in API responses — only boolean indicators
(`has_oauth2_secret`, `has_api_key`, `has_custom_headers`).
- **Field redaction for non-admins**: `convertMCPServerConfigRedacted`
strips `OAuth2ClientID`, auth URLs, scopes, and `APIKeyHeader` from
non-admin responses.
- **dbcrypt encryption at rest**: All 5 secret fields use `dbcrypt_keys`
encryption with full encrypt-on-write / decrypt-on-read wrappers (11
dbcrypt method overrides + 2 helpers), following the same pattern as
`chat_providers.api_key`.
- **OAuth2 CSRF protection**: State parameter stored in `HttpOnly`
cookie with `HTTPCookies.Apply()` for correct `Secure`/`SameSite` behind
TLS-terminating proxies.
- **dbauthz authorization**: All 18 querier methods have authorization
wrappers. Read operations use `ActionRead`, write operations use
`ActionUpdate` on `ResourceDeploymentConfig`.

## Governance Model

| Control | Implementation |
|---------|---------------|
| **Global kill switch** | `enabled` defaults to `false` |
| **Availability policy** | `force_on` (always injected), `default_on`
(pre-selected), `default_off` (opt-in) |
| **Per-chat selection** | `mcp_server_ids` on `CreateChatRequest` /
`CreateChatMessageRequest` |
| **Auth gate** | OAuth2 servers require per-user auth before tools are
injected |
| **Tool-level allow/deny** | Arrays on `mcp_server_configs` for
granular tool filtering |
| **Secrets encrypted at rest** | Uses `dbcrypt_keys` (same pattern as
`chat_providers.api_key`) |

## Tests

8 test functions covering:
- Full CRUD lifecycle (create, list, update, delete)
- Non-admin visibility filtering (enabled-only, field redaction)
- `auth_connected` population for OAuth2 vs non-OAuth2 servers
- Availability policy validation (valid values + invalid rejection)
- Unique slug enforcement (409 Conflict)
- OAuth2 disconnect idempotency
- Chat creation with `mcp_server_ids` persistence

## Known Limitations (Deferred)

These are documented and intentional for an experimental feature:
- **Audit logging** not yet wired — will add when feature stabilizes
- **Cross-field validation** (e.g., OAuth2 fields required when
`auth_type=oauth2`) — admin-only endpoint, will add when stabilizing
- **`force_on` auto-injection** — query exists but not yet wired into
chatd tool injection (follow-up)
- **Additional test coverage** — 403 auth tests, GET-by-ID tests,
callback CSRF tests planned for follow-up

## What's NOT in this PR

- Frontend UI (admin panel + chat picker)
- Actual MCP client connections (`chatd/chatmcp/` manager)
- Tool injection into `chatloop/`

coderd/chatd/chatd.go

@@ -379,6 +379,7 @@ type CreateOptions struct {
 	ChatMode           database.NullChatMode
 	SystemPrompt       string
 	InitialUserContent []codersdk.ChatMessagePart
+	MCPServerIDs       []uuid.UUID
 }
 
 // SendMessageBusyBehavior controls what happens when a chat is already active.
@@ -401,6 +402,7 @@ type SendMessageOptions struct {
 	Content       []codersdk.ChatMessagePart
 	ModelConfigID *uuid.UUID
 	BusyBehavior  SendMessageBusyBehavior
+	MCPServerIDs  *[]uuid.UUID
 }
 
 // SendMessageResult contains the outcome of user message processing.
@@ -450,6 +452,12 @@ func (p *Server) CreateChat(ctx context.Context, opts CreateOptions) (database.C
 	if len(opts.InitialUserContent) == 0 {
 		return database.Chat{}, xerrors.New("initial user content is required")
 	}
+	// Ensure MCPServerIDs is non-nil so pq.Array produces '{}'
+	// instead of SQL NULL, which violates the NOT NULL column
+	// constraint.
+	if opts.MCPServerIDs == nil {
+		opts.MCPServerIDs = []uuid.UUID{}
+	}
 
 	var chat database.Chat
 	txErr := p.db.InTx(func(tx database.Store) error {
@@ -465,6 +473,7 @@ func (p *Server) CreateChat(ctx context.Context, opts CreateOptions) (database.C
 			LastModelConfigID: opts.ModelConfigID,
 			Title:             opts.Title,
 			Mode:              opts.ChatMode,
+			MCPServerIDs:      opts.MCPServerIDs,
 		})
 		if err != nil {
 			return xerrors.Errorf("insert chat: %w", err)
@@ -596,6 +605,17 @@ func (p *Server) SendMessage(
 			modelConfigID = *opts.ModelConfigID
 		}
 
+		// Update MCP server IDs on the chat when explicitly provided.
+		if opts.MCPServerIDs != nil {
+			lockedChat, err = tx.UpdateChatMCPServerIDs(ctx, database.UpdateChatMCPServerIDsParams{
+				ID:           opts.ChatID,
+				MCPServerIDs: *opts.MCPServerIDs,
+			})
+			if err != nil {
+				return xerrors.Errorf("update chat mcp server ids: %w", err)
+			}
+		}
+
 		existingQueued, err := tx.GetChatQueuedMessages(ctx, opts.ChatID)
 		if err != nil {
 			return xerrors.Errorf("get queued messages: %w", err)