fix: #1370 Custom Headers now are visible when editing saved servers

[aakashH242]

Signed-off-by: Aakash [email protected]

🐛 Bug-fix PR

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📌 Summary

Editing a gateway saved with “Custom Headers” auth now shows the previously saved headers again. At the same time we keep other auth fields stable and mask actual secrets to avoid accidental leakage.

🔁 Reproduction Steps

#1370

🐞 Root Cause

The admin template re-renders the modal from the REST response, but the response only included the encoded auth_value and legacy single-header fields.
Because the encoded value was never decoded for the UI, auth-headers-container-gw-edit stayed empty, so the Custom Headers section appeared blank.

💡 Fix Description

• Extended GatewayRead to emit the decoded header list (plus unmasked copies for the UI) while still masking the user-facing strings (mcpgateway/schemas.py (line 3148), mcpgateway/schemas.py (line 3188)).
• Reworked the edit modal to call loadAuthHeaders("auth-headers-container-gw-edit", gateway.authHeadersUnmasked, { maskValues: true }), which rehydrates each header row and remembers the real value behind the mask (mcpgateway/static/admin.js (line 4335), mcpgateway/static/admin.js (line 11580), mcpgateway/static/admin.js (line 11822)).
• When saving, the server now preserves unchanged secrets by merging masked placeholders back with the stored values in GatewayService.update_gateway (mcpgateway/services/gateway_service.py (line 1194)).
• Added regression coverage to prove the schema surfaces the unmasked headers and that updates keep masked rows intact (tests/unit/mcpgateway/test_multi_auth_headers.py (line 20), tests/unit/mcpgateway/test_multi_auth_headers.py (line 262)).

🧪 Verification

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

✅ Checklist

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

Evidences

• Create a connection with Custom Headers.

• Edit the created connection - the headers are now visible.

[aakashH242]

Hi @crivetimihai requesting your review on this PR. Happy to address any feedback you have.