Finish implementing byte-based flow control

[graydon]

A followup to #4353 to flow-control total bytes not just flood bytes.

[Copilot]

Pull request overview

Extends byte-based flow control to throttle all inbound traffic, not only flood messages.

Changes:

• Adds configurable total byte-reading capacity with safe read headroom.
• Updates flow-control accounting, APIs, diagnostics, and tests.
• Adds repeated TCP throttling/recovery coverage for non-flood traffic.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/overlay/test/OverlayTests.cpp	Tests total-byte throttling and updated flood capacity.
src/overlay/test/LoopbackPeer.cpp	Uses renamed flood-byte capacity API.
src/overlay/Peer.cpp	Resumes reads when capacity becomes available.
src/overlay/OverlayManagerImpl.h	Renames capacity interface implementation.
src/overlay/OverlayManagerImpl.cpp	Renames flood-byte capacity calculation.
src/overlay/OverlayManager.h	Renames public capacity interface.
src/overlay/FlowControlCapacity.h	Makes total capacity mandatory and defines read floor.
src/overlay/FlowControlCapacity.cpp	Implements total byte accounting and throttling.
src/overlay/FlowControl.cpp	Initializes and reports total byte capacity.
src/main/Config.h	Declares total byte-reading configuration.
src/main/Config.cpp	Adds default and configuration parsing.
docs/stellar-core_example.cfg	Documents the new setting, but uses an incorrect default.