fix(skilltoolset): use name arg in load_skill instruction

[Zhang-986]

Summary

• correct the default SkillToolset instruction to call load_skill with name="<SKILL_NAME>", matching LoadSkillArgs
• add regression assertions so ProcessRequest no longer reintroduces the stale skill_name argument for load_skill

Fixes #912.

Testing

• go test ./tool/skilltoolset/...
• go test ./... was also run; it currently fails in google.golang.org/adk/server/adkrest/internal/services on TestDebugTelemetryGetSpansBySessionID due span ordering mismatches unrelated to this tool/skilltoolset change. Re-running that package with -count=3 reproduces the same ordering failure.

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