RCORE-2204: Use accepting_clients key to check for initial sync completion in flx tests

[kmorkos]

What, How & Why?

The initial sync progress endpoint has an additional key, accepting_clients, which in FLX is only set to true after the first initial sync completes. This is a more reliable way to check for initial sync completion than relying on individual namespaces. This PR updates the test helper to check that value instead for FLX apps.

Additionally fixing a bug in the role change tests' API calls that is manifesting in the latest baas, but not the pinned baasaas version for some reason.

☑️ ToDos

• 📝 Changelog update
• 🚦 Tests (or not relevant)
• C-API, if public C++ API changed
• bindgen/spec.yml, if public C++ API changed

[michael-wb]

LGTM

[coveralls-official]

Pull Request Test Coverage Report for Build kiro.morkos_3058

Details

• 8 of 8 (100.0%) changed or added relevant lines in 3 files are covered.
• 84 unchanged lines in 12 files lost coverage.
• Overall coverage increased (+0.007%) to 91.017%

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Files with Coverage Reduction	New Missed Lines	%
src/realm/object-store/sync/sync_session.cpp	1	92.01%
src/realm/sync/network/websocket.cpp	1	71.79%
src/realm/sync/noinst/protocol_codec.hpp	3	74.0%
src/realm/table.cpp	3	90.59%
src/realm/util/future.hpp	3	95.94%
src/realm/util/assert.hpp	4	87.1%
test/object-store/util/sync/baas_admin_api.cpp	5	84.03%
src/realm/sync/network/network.hpp	7	85.28%
src/realm/sync/instruction_applier.cpp	8	68.01%
src/realm/sync/noinst/client_impl_base.cpp	9	81.97%

Totals
Change from base Build 2507:	0.007%
Covered Lines:	216407
Relevant Lines:	237766

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💛 - Coveralls