Add unit test coverage for seven small pyamber gaps

[aglinxinyuan]

Task Summary

Seven pyamber files each carry two or three uncovered statements — individually too small to be worth an issue, collectively the bulk of what is left in the Python tree:

File	Missed	Coverage
core/architecture/handlers/control/open_executor_handler.py	2 of 6	67%
core/models/schema/schema.py	3 of 48	94%
core/models/schema/attribute_type.py	2 of 46	96%
core/models/table.py	2 of 42	95%
core/proxy/proxy_client.py	2 of 32	94%
core/runnables/network_sender.py	2 of 46	96%
core/storage/iceberg/iceberg_utils.py	2 of 98	98%

Two of these have no test module at all (open_executor_handler.py, attribute_type.py).

The uncovered lines are mostly type-dispatch failure arms and error paths — the branches taken when a caller passes something the function does not support.

Task Type

• Testing / QA

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Generated-by: Claude Code (Opus 5)