Remove dead CacheSourceOpExec and OpExecSource plumbing (replaced by input-port materialization readers in #3425) #6125

Task Summary

Cache source operators were replaced by input-port materialization reader threads in #3425 (the symmetric follow-up to sink-operator removal in #3312). Since then CacheSourceOpExec has been unreachable: the scheduler no longer inserts a cache-source read op for a materialized link — it tags the downstream input port with the URI directly.

SpecialPhysicalOpFactory.newSourcePhysicalOp — the only producer of an OpExecSource init-info, which is what constructs CacheSourceOpExec in DataProcessorRPCHandlerInitializer.setupExecutor — has no production caller; its only remaining references are its own spec. This is dead code and should be removed.

Materialized-link read path, before → after:

Before (pre-#3425):  upstream OutputPort(URI) ──► [CacheSourceOpExec] ──► downstream InputPort
After  (#3425+):     upstream OutputPort(URI) ─────────────────────────► downstream InputPort(URI)

Removal scope:

Notes:

• The OpExecInitInfo oneof is an internal controller↔worker wire format (built and consumed within the same deployment; preserve_unknown_fields: false), so dropping the field is safe — just reserve 3.
• The unit spec CacheSourceOpExecSpec was already dropped in test(workflow-operator): add unit test coverage for interval-join, and sleep #6098 (this dead-code discovery came out of that review), so nothing test-side blocks the removal.

Task Type

• Refactor / Cleanup