feat(connlib): create flow on ICMP error "prohibited" (#10462)

In Firezone, a Client requests an "access authorization" for a Resource
on the fly when it sees the first packet for said Resource going through
the tunnel. If we don't have a connection to the Gateway yet, this is
also where we will establish a connection and create the WireGuard
tunnel.

In order for this to work, the access authorization state between the
Client and the Gateway MUST NOT get out of sync. If the Client thinks it
has access to a Resource, it will just route the traffic to the Gateway.
If the access authorization on the Gateway has expired or vanished
otherwise, the packets will be black-holed.

Starting with #9816, the Gateway sends ICMP errors back to the
application whenever it filters a packet. This can happen either because
the access authorization is gone or because the traffic wasn't allowed
by the specific filter rules on the Resource.

With this patch, the Client will attempt to create a new flow (i.e.
re-authorize) traffic for this resource whenever it sees such an ICMP
error, therefore acting as a way of synchronizing the view of the world
between Client and Gateway should they ever run out of sync.

Testing turned out to be a bit tricky. If we let the authorization on
the Gateway lapse naturally, we portal will also toggle the Resource off
and on on the Client, resulting in "flushing" the current
authorizations. Additionally, it the Client had only access to one
Resource, then the Gateway will gracefully close the connection, also
resulting in the Client creating a new flow for the next packet.

To actually trigger this new behaviour we need to:

- Access at least two resources via the same Gateway
- Directly send `reject_access` to the Gateway for this particular
resource

To achieve this, we dynamically eval some code on the API node and
instruct the Gateway channel to send `reject_access`. The connection
stays intact because there is still another active access authorization
but packets for the other resource are answered with ICMP errors.

To achieve a safe roll-out, the new behaviour is feature-flagged. In
order to still test it, we now also allow feature flags to be set via
env variables.

Resolves: #10074

---------

Co-authored-by: Mariusz Klochowicz <mariusz@klochowicz.com>

.github/workflows/_integration_tests.yml

@@ -101,6 +101,8 @@ jobs:
       fail-fast: false
       matrix:
         test:
+          - script: create-flow-from-icmp-error
+            min_client_version: 1.5.4
           - script: curl-api-down
           - script: curl-api-restart
           - script: curl-ecn
@@ -128,6 +130,15 @@ jobs:
       - uses: ./.github/actions/ghcr-docker-login
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
+      - name: Check minimum client version
+        id: version_check
+        if: ${{ matrix.test.min_client_version }}
+        continue-on-error: true
+        run: |
+          ACTUAL_VERSION=$(docker run ${{ inputs.client_image }}:${{ inputs.client_tag }} firezone-headless-client --version | awk '{print $2}')
+          MIN_VERSION="${{ matrix.test.min_client_version }}"
+
+          [ "$(printf '%s\n' "$MIN_VERSION" "$ACTUAL_VERSION" | sort --version-sort | head -n1)" == "$MIN_VERSION" ]
       # We need at least Docker v28.1 which is not yet available on GitHub actions runners
       - uses: docker/setup-docker-action@b60f85385d03ac8acfca6d9996982511d8620a19 # v4.3.0
       - name: Seed database
@@ -178,6 +189,8 @@ jobs:
           sudo ethtool -K docker0 tx off
 
       - run: ./scripts/tests/${{ matrix.test.script }}.sh
+        if: ${{ steps.version_check.outcome != 'failure' }} # Run the script if version check succeeds or is skipped
+
       - name: Ensure Client emitted no warnings
         if: "!cancelled()"
         run: |