Keep subagent localProtocolOptions on their ToolSession instead of installing
them as the process-global LocalProtocolHandler override. No-context URL
consumers therefore retain the active top-level session's mapping while tan
and task subagents continue to resolve through their caller context.
Add SDK regression coverage proving subagent creation preserves an existing
global mapping.
Fixes#6971
(cherry picked from commit a02eef174b03036dc960c842a0901d22333ad9cd)
roboomp
dd4985f31e