Review and update the MQTT source for Source Developer Guide compliance

The **MQTT source** (`components/sources/mqtt`, crate `drasi-source-mqtt`) needs to be thoroughly reviewed and updated to ensure it is fully compliant with the latest requirements, recommendations, standards, and best practices defined in the [Source Developer Guide](https://github.com/drasi-project/drasi-core/blob/main/components/sources/README.md) (`components/sources/README.md`). ## Scope - Thoroughly review the entire `mqtt` source implementation against the Source Developer Guide and bring it into full compliance with the latest requirements, recommendations, standards, and best practices. - This update **may be a breaking change** — the updated source does **not** need to maintain backward compatibility with previous versions. - **No migration accommodation** is required. - All documentation and code comments **must not** refer to any previous versions of the source. - The updated source must be **fully tested and validated** to confirm it meets the latest requirements, recommendations, standards, and best practices defined in the Source Developer Guide. ## Acceptance criteria - [ ] The source is fully compliant with the Source Developer Guide (`components/sources/README.md`). - [ ] Any breaking changes required for compliance are made without backward-compatibility shims or migration accommodations. - [ ] No documentation or code comments reference previous versions of the source. - [ ] The source builds and its unit and integration tests pass with lints clean. - [ ] The source is fully tested and validated against the Source Developer Guide.