fix: prevent GPS stream from blocking HA startup (1.2.1)

[yellowcooln]

Summary

• Run the persistent GPS SSE listener as a config-entry background task so it does not block Home Assistant startup wrap-up
• Preserve explicit cancellation and deterministic runtime restart behavior
• Update the manifest and changelog to 1.2.1, crediting the report in GPS stream loop blocks HA startup wrap-up for ~5 minutes (tracked task instead of background task) #16
• Add regression coverage for background-task ownership, duplicate starts, cancellation, and restart

Verification

• Regression test failed before the fix and passed afterward
• All 104 local tests passed
• Compilation, JSON validation, and git diff --check passed
• Python Smoke, HACS, and Hassfest passed for the head commit
• Tests execute coordinator methods with fake HA boundaries; live HA startup timing has not been retested

Related issue: #16