fix(mcp-server): make mcp-server lifespan and startup non-blocking for marimo server (#6545)

## 📝 Summary

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#123).
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Update lifespan function for mcp-server to handle errors rather than
raising them to the parent server.

Now if an error occurs it won't block marimo backend server from
starting, but rather just log it like this:

<img width="896" height="277" alt="Screenshot 2025-09-26 at 7 33 42 PM"
src="https://github.com/user-attachments/assets/dcaa5081-c6c8-4453-851b-e7ac5403c6ba"
/>


## 🔍 Description of Changes

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problem addressed and how it was resolved. If applicable, provide before
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- Update except block in marimo/_mcp/server/lifespan.py to handle
unknown errors internally rather than raising it to parent Starlette
app.

## 📋 Checklist

- [x] I have read the [contributor
guidelines](https://github.com/marimo-team/marimo/blob/main/CONTRIBUTING.md).
- [ ] For large changes, or changes that affect the public API: this
change was discussed or approved through an issue, on
[Discord](https://marimo.io/discord?ref=pr), or the community
[discussions](https://github.com/marimo-team/marimo/discussions) (Please
provide a link if applicable).
- [ ] I have added tests for the changes made.
- [x] I have run the code and verified that it works as expected.

Author: bjoaquinc

marimo/_mcp/server/lifespan.py

@@ -28,4 +28,5 @@ async def mcp_server_lifespan(app: Starlette) -> AsyncIterator[None]:
         return
     except Exception as e:
         LOGGER.error(f"Failed to start MCP server: {e}")
-        raise
+        yield
+        return