fix: avoid nil interface panic on Keycloak config discovery

[pulkitvats2007-crypto]

Description

• Fixes a critical runtime panic in GetKeycloakURL() when interacting with a Microcks server that has Keycloak disabled.
• Previously, the CLI performed unconditional type assertions on auth-server-url and realm fields, which are absent when the /api/keycloak/config endpoint returns { "enabled": false }. This caused a fatal panic:
  panic: interface conversion: interface {} is nil, not string.
• Updated logic to first check the enabled flag and only access Keycloak-specific fields when it is true.
• Ensures the CLI returns "null" safely when Keycloak is disabled instead of crashing.

Implementation Details

enabled := configResp["enabled"].(bool)

if enabled {
    authServerURL := configResp["auth-server-url"].(string)
    realmName := configResp["realm"].(string)
    return authServerURL + "/realms/" + realmName + "/", nil
}
return "null", nil

Testing & Prevention

• Added unit tests in pkg/connectors/microcks_client_test.go using httptest.Server.
• Covered both scenarios:
  • Keycloak enabled → validates correct URL construction.
  • Keycloak disabled → verifies no panic and correct "null" return.
• All tests pass successfully.

Impact

• Prevents fatal runtime crashes during CLI operations such as:
  • microcks login
  • microcks test
  • Token refresh flows
• Enables seamless usage of the CLI with Microcks deployments where Keycloak is intentionally disabled (common in local setups and CI/CD pipelines).
• Eliminates a class of interface conversion panics caused by unsafe type assertions on missing JSON fields.
• Improves overall CLI robustness and production reliability.

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Related issue(s)

Fixes #267

[github-actions]

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