Split print_test_output() into focused helpers

[MatthewMckee4]

Summary

print_test_output() was a 74-line function in karva/src/commands/test/mod.rs that mixed four concerns: diagnostic printing, duration display, blank-line spacing decisions, and result summary formatting. This made the spacing logic especially hard to follow, since blank lines depended on what the other sections had printed.

This PR extracts two focused helpers:

• print_diagnostics_section() — prints discovery diagnostics and regular diagnostics, each with their concise-mode trailing newline. The caller no longer needs to know about the internal structure of the two diagnostic blocks.
• print_durations_section() — prints the N-slowest-tests table and returns bool indicating whether anything was printed. This lets the caller's spacing logic read as a simple condition (!has_diagnostics && !durations_printed) instead of recomputing the durations check inline.

With these extracted, print_test_output() shrinks to a clear sequence: leading blank line (if diagnostics), diagnostics, durations, trailing blank line (if nothing else printed one), result summary.

Also simplifies test() by resolving num_workers in one step (folding the no_parallel check into the fallback) and moving the --watch/--dry-run conflict check before the local variable extraction block.

Terminal output is unchanged — all 685 snapshot and integration tests pass.

Test plan

• just test — all 685 tests pass
• uvx prek run -a — all pre-commit checks pass (including clippy)
• No snapshot changes, confirming terminal output is identical

🤖 Generated with Claude Code

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

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For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 1 untouched benchmark

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_{Comparing refactor/print-test-output-split (7940340) with main (1514ec8)}