docs(spec): fix missing contextId in streaming example (#2083)

## What changed
- add `contextId` to the streaming task example in section 6.2
- keep the SSE example aligned with the documented event/task shape

## Why
The streaming example omitted `contextId` even though the spec requires
it for task/event continuity. This makes the example easy to copy
incorrectly.

## Root cause
The prose and field definitions were updated, but the SSE example in
`docs/specification.md` was left behind.

## Validation
- ran `bash scripts/sort_spelling.sh`
- ran `npx markdownlint-cli 'docs/**/*.md' --config
'.github/linters/.markdownlint.json' --fix`
- ran `git diff --check`

Fixes #1683

Co-authored-by: Sampath Kumar <sam1990kumar@gmail.com>

Author: juanferrub

docs/specification.md

@@ -1373,11 +1373,11 @@ Authorization: Bearer token
 HTTP/1.1 200 OK
 Content-Type: text/event-stream
 
-data: {"task": {"id": "task-uuid", "status": {"state": "TASK_STATE_WORKING"}}}
+data: {"task": {"id": "task-uuid", "contextId": "context-uuid", "status": {"state": "TASK_STATE_WORKING"}}}
 
-data: {"artifactUpdate": {"taskId": "task-uuid", "artifact": {"parts": [{"text": "# Climate Change Report\n\n"}]}}}
+data: {"artifactUpdate": {"taskId": "task-uuid", "contextId": "context-uuid", "artifact": {"parts": [{"text": "# Climate Change Report\n\n"}]}}}
 
-data: {"statusUpdate": {"taskId": "task-uuid", "status": {"state": "TASK_STATE_COMPLETED"}}}
+data: {"statusUpdate": {"taskId": "task-uuid", "contextId": "context-uuid", "status": {"state": "TASK_STATE_COMPLETED"}}}
 ```
 
 ### 6.3. Multi-Turn Interaction