Adopt Paho MQTT v2 callbacks

[Carter Tinney (cartertinney)]

Checklist

• I have read the contribution guidelines.
• I added or modified tests to cover the change.
• Public SDK behavior is unchanged, so no devdoc update is required.

Reference/Link to the issue solved with this PR (if any)

None.

Description of the problem

The MQTT transport opted into deprecated Paho callback API v1, producing a deprecation warning for customers on Paho 2.x. It also deferred Paho reconnects with an arbitrary two-hour delay, wrote to the private _thread field, and treated every successful subscribe() call as eventual subscription success even when the broker rejected the SUBACK.

Description of the solution

• Use callback API v2 for TCP and WebSocket clients and update every callback signature.
• Document that Paho synthesizes MQTT 5 ReasonCode and Properties callback values for MQTT 3.1.1; keep those Paho API concepts at the callback boundary.
• Classify v2 CONNACK and disconnect ReasonCode objects by documented semantic names while preserving existing SDK exception classification.
• Complete rejected SUBACKs with ProtocolClientError; retain completion errors in OperationManager when an ACK races ahead of operation establishment.
• Set reconnect_on_failure=False, remove the reconnect-delay workaround, and rely on Paho 2.1 callback-safe loop_stop() and _thread_main cleanup instead of assigning _thread directly.
• Require paho-mqtt>=2.1.0,<3.0.0 and update uv.lock.
• Add public-client warning-as-error coverage and focused callback, disconnect, cleanup, and early-ACK tests.

Validation

• Targeted transport, MQTT pipeline-stage, and synchronous client tests: 1213 passed, 3 skipped
• Black passed
• Ruff passed
• git diff --check passed