fix(controller): return Unimplemented from GetChangedTargetGraph stub (audit #5)

[sbalabanov]

Summary

GetChangedTargetGraph was a silent stub that returned nil (empty success stream) while the proto contract promises a full streamed changed target graph. Metrics reported success, making it impossible to distinguish the stub from a real call.

• Return a YARPC Unimplemented error classified as ErrorUser so clients get an explicit failure
• Metrics lifecycle now reflects the failure: finish histogram tagged result=user, failure counter emitted with error_code=user
• Proto RPC comment annotated with "NOT YET IMPLEMENTED"
• Regenerated tangopbmock to include TangoServiceGetChangedTargetGraphYARPCServer
• Added table-driven test asserting the YARPC error code and that no chunks are streamed
• Updated existing metrics path-shape test to expect the new failure outcome

Test plan

• go build ./... passes
• go test ./controller/... passes (new + updated tests)
• make gazelle BUILD files in sync

🤖 Generated with Claude Code

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