fix(admin): allow clearing passthrough_headers via gateway edit form

[ecthelion77]

🐛 Bug-fix PR

Fixes #4228

📌 Summary

Passthrough headers cannot be cleared once set on a gateway via the admin edit form. Clearing the field and saving silently preserves the old values.

🔁 Reproduction Steps

1. Create a gateway with passthrough_headers set (e.g. ["Authorization"])
2. Edit the gateway, clear the passthrough_headers field
3. Save — the field still shows the previous values

🐞 Root Cause

Two bugs:

1. admin.py: str(form.get("passthrough_headers")) converts empty/None to the string "None", which is truthy. The parsing logic treats it as valid content instead of a cleared field, so passthrough_headers is never set to signal clearing.

2. gateway_service.py: Even if an empty list [] is passed (meaning "user cleared the field"), it is stored as-is in the database. The canonical representation for "no passthrough headers" is None, not [].

💡 Fix Description

• admin.py: Distinguish between "field cleared" (empty string → set passthrough_headers = [] to signal clearing) and "field not provided" (None → no change). Avoid str() coercion on raw form value.

• gateway_service.py: Convert empty list [] to None before storing, so the DB always uses None to mean "no passthrough headers".

🧪 Verification

Check	Command	Status
Lint suite	make lint	✅
Unit tests	make test	✅
Manual regression no longer fails	Clear field, save, verify cleared	✅

📐 MCP Compliance

• No change to MCP protocol behavior
• No breaking change to MCP clients

✅ Checklist

• Code formatted
• No secrets/credentials committed
• DCO Signed-off-by included