pc: recover congestion callback and eoc shim work

[thesyncim]

Summary

• recover the unlanded congestion-state callback work from the automation loop
• add GetCongestionState() and SetOnCongestionState() plus focused end-to-end coverage in pkg/pc
• teach the shim source to accept browser-style empty ICE candidates as end-of-candidates markers instead of failing the parse path

Validation

• GOCACHE=/tmp/libgowebrtc-pc-pr-gocache LIBWEBRTC_SHIM_PATH=/Users/thesyncim/GolandProjects/libgowebrtc/lib/darwin_arm64/libwebrtc_shim.dylib go test ./pkg/pc -run 'Test(AddICECandidateAcceptsEndOfCandidates|GetCongestionState|SetOnCongestionState)$' -count=1
• Current result: fails on TestAddICECandidateAcceptsEndOfCandidates with add ice candidate failed because the checked-in source fix in shim/shim_peer_connection.cc is not present in the existing local dylib yet.

Follow-up

• rebuild the darwin-arm64 shim with Bazel/Xcode and rerun the focused pkg/pc tests before merge