fix(api): restrict MQTT sample endpoint to admins and disallow wildcard topics

[Micsi]

Motivation

• The GET /instances/{instance_id}/mqtt/sample endpoint previously allowed any authenticated user to subscribe with server-side adapter credentials to an arbitrary topic, which could disclose sensitive broker payloads.
• Wildcard topics and empty topics increase the risk of broad or unintended data disclosure when the server subscribes on behalf of callers.

Description

• Require admin-level authorization for the sampling endpoint by switching the dependency from get_current_user to get_admin_user in obs/api/v1/adapters.py for mqtt_sample_payload to prevent low-privileged API keys from invoking it.
• Add topic validation in obs/api/v1/adapters.py to strip() the topic, reject empty topics, and return HTTP 400 if the topic contains MQTT wildcard characters # or +.
• Preserve existing behavior and timeout handling for single-topic sampling and error handling when aiomqtt is unavailable or the MQTT connection fails.

Testing

• Ran python -m compileall obs/api/v1/adapters.py and the module compiled successfully with no syntax errors.

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Codex Task