Part of #953

What

Raise line coverage of src/tracing/exporters.cpp from 7.4% to >= 70%, and branch coverage to >= 60%.

Current state (pre-Step-1 snapshot, 2026-04-13 measurement from parent epic #953):

• Lines hit: 25 / 340 (7.4%)
• File size: 340 LOC

Note: These numbers predate the BUILD_WITH_* workflow fix (#987, #989). A re-measurement on the current develop baseline will be captured in the PR description before merging.

Why

• Tracing exporters are the egress path for span data; silent failures here mean missing observability in production with no on-host signal.
• 7.4% line coverage implies exporter error paths (network failures, backend 5xx, serialization errors, shutdown races) are largely unexercised.
• Observability code is particularly costly to debug in production — regression bugs surface only when tracing is most needed.

Where

• Source: src/tracing/exporters.cpp
• Headers: include/kcenon/network/tracing/exporters.hpp (verify exact path)
• Existing tests to extend: tests/unit/tracing/ — check for existing exporter tests before adding
• May require: test doubles for exporter sinks (OTLP/HTTP/stdout backends) — inspect existing fixtures first

How

Approach

1. Enumerate each concrete exporter implementation (OTLP, HTTP, stdout, noop — verify actual set in code).
2. Write tests covering:
   • Happy-path export of a populated span batch
   • Empty-batch and oversized-batch handling
   • Backend-failure paths (connection refused, timeout, 5xx response, partial write)
   • Serialization edge cases (unicode span names, empty attributes, deeply nested events)
   • Shutdown behavior: pending exports flushed, in-flight exports cancelled safely
3. Use existing tracing test fixtures if present; otherwise add minimal sink doubles.

Acceptance Criteria

• src/tracing/exporters.cpp line coverage >= 70%
• src/tracing/exporters.cpp branch coverage >= 60%
• New or extended tests in tests/unit/tracing/
• All tests pass on Ubuntu/macOS/Windows CI
• ASAN/TSAN/UBSAN all green on the new tests
• Current develop baseline re-measured in PR description

Dependencies

• Parent: Expand unit test coverage from 40% to 80% #953
• Prerequisite: Step 1 (PR ci(coverage): enable full-ecosystem coverage build #987, ci(coverage): enable messaging_bridge in coverage build #989) — merged.