Closes #1034

What

Adds tests/unit/unified_session_manager_extra_coverage_test.cpp —
a new gtest binary that targets branches and edge cases of
src/core/unified_session_manager.cpp not exercised by the existing
unified_session_manager_test.cpp.

Why

Issue #1034 (part of #953) requires raising line coverage of
src/core/unified_session_manager.cpp to >= 70% and branch coverage
to >= 60%. The pre-Step-1 baseline reported 26.1% line coverage (2026-04-13).
The uncovered region concentrates in error and boundary paths — exactly
the surfaces that regress during the ongoing Result<T> migration —
so this PR fills in those paths with hermetic unit tests.

Where

• Source under test: src/core/unified_session_manager.cpp
• New test file: tests/unit/unified_session_manager_extra_coverage_test.cpp
• Build wiring: tests/CMakeLists.txt (registers
  network_unified_session_manager_extra_coverage_test via
  add_network_test)
• Changelog: CHANGELOG.md and docs/CHANGELOG.md (Tests / Added)

How

New tests close the following previously-uncovered paths:

• Default-constructed manager configuration defaults
• is_backpressure_active() short-circuit when enable_backpressure is false
• get_utilization() divide-by-zero guard when max_sessions == 0
• add_session(session_handle, id) overload (pre-wrapped handle path,
  both explicit id and auto-generated id branches)
• add_session_with_id rejection at the limit / explicit id passthrough /
  collision returning empty / auto-generated id prefix
• Duplicate-id add_session does not inflate total_accepted
• update_activity and remove_session returning false on missing ids
• cleanup_idle_sessions no-op when nothing is idle (and on empty manager)
• stop_all_sessions and clear_all_sessions equivalence and reusability
• for_each over empty manager (mutable + const overloads)
• generate_id uniqueness across threads (4 threads x 200 ids each)
• set_max_sessions narrowing below current count blocks new accepts
  and toggles backpressure
• Full stats population including idle_timeout and backpressure_active
• Const get_session round-trip with type recovery
• total_cleaned_up accumulation across multiple cleanup invocations

The source under test is not modified. No source refactoring or seam
insertion was needed — all branches are reachable through the public API.
Tests are hermetic: no network, no filesystem, only short sleep_for calls
(<= 50ms) for activity-driven cleanup paths, mirroring the existing
unified_session_manager_test.cpp style.

Test Plan

• New file compiles with the rest of the test suite via gtest_main
• No source changes to src/core/unified_session_manager.cpp
• Conventional Commits format, English-only commits / PR / changelog
• CI: Ubuntu / macOS / Windows multi-platform unit tests
• CI: ASAN / TSAN / UBSAN sanitizer suites
• CI: Coverage workflow confirms src/core/unified_session_manager.cpp
  reaches >= 70% line / >= 60% branch

Notes

• Local C++ toolchain (cmake/ninja) is unavailable in this environment,
  so the build was not verified locally; CI is the source of truth here.
• Test target name follows the existing convention used by
  network_tcp_socket_extra_coverage_test registered for test: expand coverage for src/tcp_socket.cpp #1032.
• New test types (extra_tcp_session, extra_idle_session) live in an
  anonymous namespace inside the test TU and have local
  session_traits specializations to avoid collisions with the
  existing test_tcp_session / test_idle_session types in the
  sibling test binary.