fix(gatewayapi): sort api key auth credentials

[nobbs]

What type of PR is this?

fix

What this PR does / why we need it:

API key auth credentials are currently built by iterating over Secret.Data directly. Since Secret.Data is a Go map, the credential order can vary between otherwise equivalent translations.

That nondeterministic order is preserved into the generated API key auth filter and per-route xDS config. In clusters with frequent Gateway API/xDS rebuilds, Envoy can receive byte-different listener or route resources even when the effective API key auth configuration did not change. For long-lived connections such as WebSockets, this can cause Envoy to replace the active filter chain and close existing connections with:

downstream_local_disconnect(filter_chain_is_being_removed)

This PR makes API key auth credential generation deterministic by sorting the secret data keys before appending credentials to the IR. It also adds a unit test that verifies credentials are emitted in stable client ID order.

With this change applied in a reproducing environment, the internal HTTPS listener stopped flipping due to API key credential reordering and the WebSocket connection stayed alive.

Validation:

• make generate gen-check
• go test ./internal/gatewayapi

Which issue(s) this PR fixes:

Fixes #8889

Release Notes: Yes

[netlify]

✅ Deploy Preview for cerulean-figolla-1f9435 ready!

Name	Link
🔨 Latest commit	6b7a522
🔍 Latest deploy log	https://app.netlify.com/projects/cerulean-figolla-1f9435/deploys/6a157549fe8d610008c58c5a
😎 Deploy Preview	https://deploy-preview-9042--cerulean-figolla-1f9435.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.81%. Comparing base (34b343b) to head (ecfb79d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9042      +/-   ##
==========================================
+ Coverage   74.76%   74.81%   +0.05%     
==========================================
  Files         252      252              
  Lines       40571    40576       +5     
==========================================
+ Hits        30331    30356      +25     
+ Misses       8166     8149      -17     
+ Partials     2074     2071       -3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.