egress: stop Helm clobbering serge's runtime allowlist additions (#53)

## Problem

The `serge-egress` tinyproxy filter is a single `.data.filter` string
written by **two** parties:

- **Helm** seeds the base allowlist
(`taskExecution.kubernetes.egress.allowDomains`: GitHub + the HF LLM
router).
- **serge** patches the same ConfigMap at runtime to add configured LLM
provider hosts — `reviewbot/k8s_sandbox.py:sync_egress_allowlist`, which
is additive and non-destructive (reads the current filter, appends only
missing hosts).

Under **server-side apply** the two managers can't share ownership of
one atomic string. The next `helm upgrade` re-applies the base-only
value onto serge's augmented value and fails:

```
UPGRADE FAILED: conflict occurred while applying object serge/serge-egress /v1, Kind=ConfigMap:
Apply failed with 1 conflict: conflict with "OpenAPI-Generator" using v1: .data.filter
```

This blocks **every** serge deploy once serge has added a provider host
live (e.g. `api.anthropic.com`, `api.openai.com`).

## Fix (chart-only)

Render `.data.filter` as the **union of the live filter (read via
`lookup`) and the declared base domains**:

- Helm still **guarantees the base allowlist is always present**
(appends any base domain that's missing).
- Helm **re-emits serge's runtime additions verbatim**, so its applied
value equals the live value in steady state → **no SSA conflict**.
- No serge code change, no image rebuild.

### Caveat
`lookup` returns nothing under `helm template` / `--dry-run` (no cluster
read), so a **dry-run renders only the base domains**. The real `helm
upgrade` reads the live filter and preserves serge's additions. This is
documented in a comment in the template.

## Testing
- `helm template … --show-only templates/egress-proxy.yaml` → renders
the base 3 domains, no errors (expected: `lookup` empty under
`template`).

Author: tarekziade

deploy/helm/templates/egress-proxy.yaml

@@ -42,7 +42,35 @@ data:
     PidFile "/tmp/tinyproxy.pid"
     LogLevel Info
   filter: |
+    {{- /*
+      Helm's job here is only to GUARANTEE the base allowlist (allowDomains) is
+      always present. serge itself patches this same ConfigMap at runtime to add
+      configured LLM provider hosts (reviewbot/k8s_sandbox.py:sync_egress_allowlist),
+      additively and at any time. Because .data.filter is a single string, Helm
+      and serge would otherwise fight over its ownership under server-side apply:
+      Helm re-applying the base-only value onto serge's augmented value conflicts.
+
+      So on upgrade we read the LIVE filter via `lookup`, re-emit it verbatim, and
+      union the base domains in (appending any that are missing). This preserves
+      serge's runtime additions and makes Helm's applied value equal the live
+      value in steady state -> no SSA conflict.
+
+      NOTE: `lookup` returns nothing under `helm template` / `--dry-run` (no cluster
+      read), so a dry-run renders ONLY the base domains. The real `helm upgrade`
+      reads the live filter and preserves serge's additions.
+    */}}
+    {{- $out := list }}
+    {{- $existing := lookup "v1" "ConfigMap" .Release.Namespace $name }}
+    {{- if $existing }}
+    {{-   range (splitList "\n" (get ($existing.data | default dict) "filter" | default "")) }}
+    {{-     $line := trim . }}
+    {{-     if and $line (not (has $line $out)) }}{{ $out = append $out $line }}{{ end }}
+    {{-   end }}
+    {{- end }}
     {{- range $egress.allowDomains }}
+    {{-   if not (has . $out) }}{{ $out = append $out . }}{{ end }}
+    {{- end }}
+    {{- range $out }}
     {{ . }}
     {{- end }}
 ---