docs: add LangDB AI Gateway as a tracing provider (#1224)

Adds LangDB AI Gateway to the External tracing processors list so
developers can stream Agent‑SDK traces directly into LangDB’s
dashboards.

## Highlights

- End‑to‑end observability of every agent step, tool invocation, and
guardrail.
- Access to 350+ LLM models through a single OpenAI‑compatible endpoint.

- Quick integration: `pip install "pylangdb[openai]" `
```python
from pylangdb.openai import init
init()

client = AsyncOpenAI(
    api_key=os.environ["LANGDB_API_KEY"],
    base_url=os.environ["LANGDB_API_BASE_URL"],
    default_headers={"x-project-id": os.environ["LANGDB_PROJECT_ID"]},
)
set_default_openai_client(client)     
```
- Live demo Thread:
https://app.langdb.ai/sharing/threads/53b87631-de7f-431a-a049-48556f899b4d
<img width="1636" height="903" alt="image"
src="https://github.com/user-attachments/assets/075538fb-c1af-48e8-95fd-ff3d729ba37d"
/>

Fixes #1222

---------

Co-authored-by: mutahirshah11 <[email protected]>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: 3 people

docs/tracing.md

@@ -117,3 +117,4 @@ To customize this default setup, to send traces to alternative or additional bac
 -   [Okahu-Monocle](https://github.com/monocle2ai/monocle)
 -   [Galileo](https://v2docs.galileo.ai/integrations/openai-agent-integration#openai-agent-integration)
 -   [Portkey AI](https://portkey.ai/docs/integrations/agents/openai-agents)
+-   [LangDB AI](https://docs.langdb.ai/getting-started/working-with-agent-frameworks/working-with-openai-agents-sdk)