fix:rename VLLM_SPYRE ENV to SENDNN_INFERENCE

[bayo-ibm]

The PR seeks to replace the naming convention for environment variables from VLLM_SPYRE* to SENDNN_INFERENCE*. This aligns with the change in the naming convention in the new VLLM version (please see link)

[WarningRan]

Hi @bayo-ibm, thanks for the rename!

Could you paste a short summary of the Slack context into the PR description? It'd help future readers (and anyone doing git blame later) to have it on GitHub rather than only in Slack. The key points worth capturing:

• Part of the broader vllm_spyre → sendnn_inference migration (Python package rename, VLLM_PLUGINS=spyre → sendnn_inference, plus the env vars in this PR).
• VLLM_DT_* backend vars are intentionally unchanged.
• Lands with the 3.5 EA1 / 2.0 swap.

One question: is this a hard cutover with 3.5 EA1, or should fmwork keep dual-name compatibility for a while? Our downstream log parsing / JSON post-processing needs to know whether to support both names.

Thanks!