fix(generator): Connection base-class operations should fail

[devbww]

While we do not test the behavior of the generated "ServiceConnection"
base classes, all operations should fail with kUnimplemented. That
was not the case for paginated calls, which previously returned an
empty sequence instead.

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #8236 (2892287) into main (6600f0f) will increase coverage by 0.01%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##             main    #8236      +/-   ##
==========================================
+ Coverage   94.95%   94.97%   +0.01%     
==========================================
  Files        1327     1327              
  Lines      119194   119169      -25     
==========================================
- Hits       113186   113185       -1     
+ Misses       6008     5984      -24     

Impacted Files	Coverage Δ
...ion_tests/golden/golden_kitchen_sink_connection.cc	45.71% <0.00%> (+5.71%)	⬆️
...tion_tests/golden/golden_thing_admin_connection.cc	28.57% <0.00%> (+7.51%)	⬆️
generator/internal/connection_generator.cc	67.08% <ø> (ø)
google/cloud/internal/pagination_range.h	100.00% <100.00%> (ø)
.../cloud/storage/benchmarks/throughput_experiment.cc	74.37% <0.00%> (-0.51%)	⬇️
google/cloud/pubsub/samples/samples.cc	92.02% <0.00%> (-0.08%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	98.00% <0.00%> (+0.24%)	⬆️

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[dbolduc]

Can we return this instead:

return google::cloud::internal::MakeUnimplementedPaginationRange<
    StreamRange<$range_output_type$>>();

google-cloud-cpp/google/cloud/internal/pagination_range.h

Lines 182 to 189 in 306d12b

	template <typename Range>
	Range MakeUnimplementedPaginationRange() {
	using ValueType = typename Range::value_type::value_type;
	return MakeStreamRange<ValueType>(
	[]() -> typename StreamReader<ValueType>::result_type {
	return Status{StatusCode::kUnimplemented, "needs-override"};
	});
	}

where maybe we change the message to "not implemented" instead of "needs-override"

[devbww]

Can we return this instead:

Done. PTAL.

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[devbww]

Note: Unfortunately the change in google/cloud/internal/pagination_range.h from "needs-override" to "not implemented" was incorrectly grouped in the "regeneration" commit ef7628a.