Add Webserver HTTPRoute support to the Helm chart 1.2x line

[aaron-y-chen]

closes: #61452

Summary

This adds Helm chart support for exposing the Webserver with Kubernetes Gateway API HTTPRoute on the 1.2x chart line (Airflow 2.x).

• adds webserver.httpRoute values, schema, template, tests, and docs;
• routes the default backend to the Webserver service on ports.airflowUI;
• supports custom labels, annotations, hostnames, parentRefs, path settings, and full rules override;
• renders only for Airflow 2 (airflowVersion < 3.0.0), where the Webserver exists — on Airflow 3 the API Server replaces it;
• fails fast when Webserver Ingress and Webserver HTTPRoute are both enabled (guarded from both the HTTPRoute and the Ingress template);
• extracts the shared httpRoute schema into definitions.httpRoute so apiServer, flower, and webserver reference one definition instead of three near-identical copies.

Validation

E2E screenshot through the Gateway API path (Envoy Gateway, Airflow 2.11):

The port-forward targets the Envoy Gateway dataplane service and the URL host airflow.localhost is the Gateway listener hostname, so the request path is:

browser -> Envoy Gateway service -> Gateway listener -> Webserver HTTPRoute -> airflow-webserver service

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Generated-by: [Claude Opus 4.8] following the guidelines

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• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.

[bujjibabukatta]

changes looks good .