Support split buffers over bridges

[dallison]

Summary

• Add bridge negotiation for split-buffer framing and independent remote bridge publisher split-buffer mode.
• Preserve legacy single-frame bridge reads/writes when split buffers are not enabled.
• Make bridge tests stable and run them in CI with split buffers both enabled and disabled.

Test plan

• bazelisk test //client:bridge_test --runs_per_test=100 --test_output=errors
• bazelisk test //client:bridge_test --test_arg=--use_split_buffers=false --test_output=errors
• bazelisk test //client:bridge_test --test_arg=--use_split_buffers=true --test_output=errors
• bazelisk test //client:bridge_test --runs_per_test=30 --test_arg=--use_split_buffers=false --test_output=errors
• bazelisk test //client:bridge_test --runs_per_test=30 --test_arg=--use_split_buffers=true --test_output=errors
• bazelisk test //client:bridge_test //shadow:shadow_test //c_client:client_test //server:server_test