feat: add REST API handlers and client methods for user secrets (#24107)

Add the five REST endpoints for managing user secrets, SDK client
methods, and handler tests.

Endpoints:
- `POST /api/v2/users/{user}/secrets`
- `GET /api/v2/users/{user}/secrets`
- `GET /api/v2/users/{user}/secrets/{name}`
- `PATCH /api/v2/users/{user}/secrets/{name}`
- `DELETE /api/v2/users/{user}/secrets/{name}`

Routes are registered under the existing `/{user}` group with
`ExtractUserParam`. The delete query was changed from `:exec` to
`:execrows` so the handler can distinguish "not found" from success
(DELETE with `:exec` silently returns nil for zero affected rows).

coderd/database/querier_test.go

@@ -7376,7 +7376,7 @@ func TestUserSecretsCRUDOperations(t *testing.T) {
 		assert.Equal(t, "WORKFLOW_ENV", updatedSecret.EnvName) // EnvName unchanged
 
 		// 6. DELETE
-		err = db.DeleteUserSecretByUserIDAndName(ctx, database.DeleteUserSecretByUserIDAndNameParams{
+		_, err = db.DeleteUserSecretByUserIDAndName(ctx, database.DeleteUserSecretByUserIDAndNameParams{
 			UserID: testUser.ID,
 			Name:   "workflow-secret",
 		})