Fix/passthrough header save

[arjavdongaonkar]

🐛 Bug-fix PR

Before opening this PR please:

1. make lint - passes ruff, mypy, pylint
2. make test - all unit + integration tests green
3. make coverage - ≥ 90 %
4. make docker docker-run-ssl or make podman podman-run-ssl
5. Update relevant documentation.
6. Tested with sqlite and postgres + redis.
7. Manual regression no longer fails. Ensure the UI and /version work correctly.

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📌 Summary

Edit Gateway API does not persist the passthrough_headers field from GatewayUpdate.
Although the field exists in the GatewayUpdate model, it is ignored during update and never stored in the DB.

🔁 Reproduction Steps

Create a Gateway Server.
Reopen the "Edit" modal.
Fill in the Passthrough Headers input and hit save
Observe that Passthrough headers are not saved
issue: #867

🐞 Root Cause

This was confirmed in the update_gateway method: while it updates name, url, tags, auth, etc., it never checks gateway_update.passthrough_headers.

💡 Fix Description

Update passthrough headers as well

🧪 Verification

Check	Command	Status
Lint suite	make lint
Unit tests	make test
Coverage ≥ 90 %	make coverage
Manual regression no longer fails	steps / screenshots

📐 MCP Compliance (if relevant)

• Matches current MCP spec
• No breaking change to MCP clients

✅ Checklist

• Code formatted (make black isort pre-commit)
• No secrets/credentials committed