fix: [sc-96140] Set explicit MQTT ConnectTimeout instead of relying on paho default

[mlataza]

Summary

The MQTT client options assembled in runCycle set AutoReconnect=false and an OnConnectionLost callback, but never called SetConnectTimeout — so initial-connect/subscribe operations silently relied on paho.mqtt.golang's implicit 30s default. This coupled our reconnect behaviour to an undeclared library default that could drift across pinned paho versions without any signal in our code.

This PR makes the connect timeout fully owned by our code so reconnect behaviour stays predictable across paho upgrades.

Changes

• internal/utils/time.go — Added InitialReconnectInterval (1s), now the seed for the reconnect backoff, plus DefaultMqttConnectTimeout as a documented constant alongside the backoff parameters. The default is <= the shortest possible backoff slot (1.5 × InitialReconnectInterval after worst-case jitter), so a connect attempt can never outlive the next backoff slot.
• internal/agent/device.go — Added optional per-device override mqtt_connect_timeout_seconds (mirroring the existing mqtt_qos tunable) and a Device.MqttConnectTimeout() helper.
• internal/mqtt/common.go — NewClientOptions now calls opts.SetConnectTimeout(...) centrally for all broker paths.
• Tests — Assert the default and per-device override in common_test.go; record the chosen timeout/rationale and prove it fits within the backoff in time_test.go.

Acceptance criteria

• ✅ ConnectTimeout explicitly set to a documented value (constant + rationale comment).
• ✅ Overridable per-device via the existing device config struct.
• ✅ Value consistent with the initial backoff interval — connect attempt never outlives the next backoff slot.
• ✅ Unit tests + comments record the chosen timeout and rationale.

Notes

The default is 1s, a behavioural reduction from paho's old 30s, chosen to strictly satisfy the "never outlives the next backoff slot" criterion. Tenants with occasionally slow TLS handshakes are covered by the new per-device mqtt_connect_timeout_seconds override rather than a looser default.

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