refactor(helm): extract JWT secret env to dedicated helper

[AaryanCdry]

Summary

This PR extracts the AIRFLOW__API_AUTH__JWT_SECRET environment variable block from the inline definition in standard_airflow_environment to a standalone jwt_secret_environment helper template in chart/templates/_helpers.yaml.

Motivation

PR #63204 added the JWT secret env var directly inside standard_airflow_environment. While this works, it means the JWT env var is inherited by every deployment template that includes the standard helper, making it impossible to include or exclude it per component.

Extracting it to a dedicated helper:

• Improves template readability by separating the JWT concern
• Enables future per-component opt-in/opt-out of the JWT env var
• Follows the existing pattern where each secret has its own named helper

Changes

-  {{- if and (semverCompare ">=3.0.0" .Values.airflowVersion) .Values.enableBuiltInSecretEnvVars.AIRFLOW__API_AUTH__JWT_SECRET }}
-  - name: AIRFLOW__API_AUTH__JWT_SECRET
-    valueFrom:
-      secretKeyRef:
-        name: {{ template "jwt_secret" . }}
-        key: jwt-secret
-  {{- end }}
+  {{- include "jwt_secret_environment" . }}

New helper added after standard_airflow_environment:

{{- define "jwt_secret_environment" }}
  {{- if and (semverCompare ">=3.0.0" .Values.airflowVersion) .Values.enableBuiltInSecretEnvVars.AIRFLOW__API_AUTH__JWT_SECRET }}
  - name: AIRFLOW__API_AUTH__JWT_SECRET
    valueFrom:
      secretKeyRef:
        name: {{ template "jwt_secret" . }}
        key: jwt-secret
  {{- end }}
{{- end }}

Behavior

No behavior change. All components that previously received the JWT env var continue to receive it through the new helper via standard_airflow_environment.

Closes #70843

[SameerMesiah97]

Looks like you might need a rebase. And why have you opened 2 PRs? Please close one.

[AaryanCdry]

Closing in favor of #70913, which contains the same refactor rebased cleanly on current main (this branch had merge conflicts). Same change, same author — keeping the mergeable version.