Update LSD control plane API auth section to specify X-Tenant-Id (#1163)

## Overview
Make it super explicit that `X-Tenant-Id` should be specified in the LSD
control plane API request. It's needed for org-scoped API keys and it
doesn't hurt to specify it for workspace-scoped API keys.

Be explicit. Reduce user support questions.

## Type of change

**Type:** Update existing documentation

## Related issues/PRs
- Slack thread:
https://langchain.slack.com/archives/C089RDWTKU4/p1761648613577899

## Checklist
<!-- Put an 'x' in all boxes that apply -->
- [x] I have read the [contributing guidelines](README.md)
- [x] I have tested my changes locally using `docs dev`
- [x] All code examples have been tested and work correctly
- [x] I have used **root relative** paths for internal links
- [x] I have updated navigation in `src/docs.json` if needed
- I have gotten approval from the relevant reviewers
- (Internal team members only / optional) I have created a preview
deployment using the [Create Preview Branch
workflow](https://github.com/langchain-ai/docs/actions/workflows/create-preview-branch.yml)

## Additional notes
N/A

Author: andrewnguonly

src/langsmith/api-ref-control-plane.mdx

@@ -21,14 +21,15 @@ The control plane hosts for Cloud data regions:
 
 ## Authentication
 
-To authenticate with the control plane API, set the `X-Api-Key` header to a valid LangSmith API key.
+To authenticate with the control plane API, set the `X-Api-Key` header to a valid LangSmith API key and set the `X-Tenant-Id` header to a valid workspace ID to target.
 
 Example `curl` command:
 
 ```shell
 curl --request GET \
   --url http://localhost:8124/v2/deployments \
   --header 'X-Api-Key: LANGSMITH_API_KEY'
+  --header 'X-Tenant-Id': WORKSPACE_ID'
 ```
 
 ## Versioning
@@ -59,6 +60,7 @@ load_dotenv()
 # required environment variables
 CONTROL_PLANE_HOST = os.getenv("CONTROL_PLANE_HOST")
 LANGSMITH_API_KEY = os.getenv("LANGSMITH_API_KEY")
+WORKSPACE_ID = os.getenv("WORKSPACE_ID")
 INTEGRATION_ID = os.getenv("INTEGRATION_ID")
 MAX_WAIT_TIME = 1800  # 30 mins
 
@@ -67,6 +69,7 @@ def get_headers() -> dict:
     """Return common headers for requests to the control plane API."""
     return {
         "X-Api-Key": LANGSMITH_API_KEY,
+        "X-Tenant-Id": WORKSPACE_ID,
     }