fix: accept interfaces as the only routing target for applications

[khshanovskyi]

Description of changes

• The application write API rejected applications whose only routing target is interfaces with 400 At least application endpoint, MCP endpoint or function must be provided — a check in ApplicationService that predates the typed interfaces map, while the same shape loaded from the config file routes normally.
• DeploymentEndpointUtil.hasRoutingInterface(deployment): the interfaces map contributes a servable type — an entry carrying a translator reference or a url of its own (the deployment-level baseUrl included), or one whose type a legacy field also serves. An entry resolving to nothing is not a routing target, mirroring the 503 the request path answers for that same shape.
• ApplicationService.prepareApplication now accepts such applications, and the validation message names the interface option.
• Tests: write-path regression in ApplicationInterfacesApiTest (an interfaces-only application is stored and serves through the interface; an entry without any url is still rejected) plus branch coverage for hasRoutingInterface in DeploymentEndpointUtilTest.

Fixes #1965

Checklist

• Title of the pull request follows Conventional Commits specification
• All tests pass (ApplicationInterfacesApiTest 5/5 including the new write-path regression, DeploymentEndpointUtilTest, application write/serving suites, checkstyle clean; full suite green apart from the known Redis-port flake, which passes on rerun)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

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