refactor: extract helpers, dedup patterns, add regression tests

Mechanical cleanup pass, behavior-preserving:
- remove dead code: reposition_bars (unused), sbar click infrastructure
  (zones never populated, handler iterated empty list)
- extract helpers in mode.lua: win_ok, buf_ok, is_dapui_pane_ft,
  for_each_dapui_pane (asymmetric filter preserved), safe_extmark,
  bar_set_lines, safe_set_height, if_still_active, invalidate_dead_bars,
  focus_source_win, unset_and_restore
- inline install_cs_listener inside define_highlights to dedup the three
  ColorScheme augroup+autocmd triples (italic, key, sep)
- data-drive sign_define in breakpoints.lua
- data-drive python interpreter candidate probing
- add tests/test_render_bars.lua pinning sbar/cbar output dimensions,
  zones, and overflow fallback behavior
- add tests/test_highlights_cs.lua verifying ColorScheme augroups and
  highlight group existence / re-apply on event
- add tests/test_helpers.lua unit-testing keyof and source_window
- expose M._test (direct function refs) so render/highlight internals are
  testable in headless mode

tests/test_render_bars.lua

@@ -0,0 +1,147 @@
+local h = require("helpers")
+local assert_eq = h.assert_eq
+local assert_true = h.assert_true
+local assert_false = h.assert_false
+
+local mode = require("turbo-debug.mode")
+local config = require("turbo-debug.config")
+
+-- Ensure config is initialized with defaults so keyof/render_cbar can read it
+config.merge({})
+
+-- Helpers --------------------------------------------------------------------
+
+local function make_bar_win()
+  vim.cmd("noautocmd keepalt topleft split")
+  local win = vim.api.nvim_get_current_win()
+  local buf = vim.api.nvim_create_buf(false, true)
+  vim.api.nvim_win_set_buf(win, buf)
+  vim.api.nvim_win_set_height(win, 2)
+  return win, buf
+end
+
+local function cleanup(win)
+  if win and vim.api.nvim_win_is_valid(win) then
+    pcall(vim.api.nvim_win_close, win, true)
+  end
+end
+
+local function line(buf, i)
+  return vim.api.nvim_buf_get_lines(buf, i, i + 1, false)[1] or ""
+end
+
+-- build_control_text -------------------------------------------------------
+
+-- Inline form when key matches first letter case-insensitively.
+local text, kcol, kend, icol, iend = mode._test.build_control_text(">", "s", "step", "over")
+assert_true(text:find("%(s%)tep") ~= nil, "inline form: (s)tep present")
+assert_true(kcol < kend, "inline form: key_col < key_end_col")
+assert_true(icol ~= nil and iend ~= nil, "inline form: italic span present")
+assert_true(text:sub(icol + 1, iend) == "over", "inline form: italic text is 'over'")
+
+-- Prefix form when key does NOT match first letter.
+local text2, _, _, icol2 = mode._test.build_control_text(">", "n", "step", "over")
+assert_true(text2:find("%(n%) step") ~= nil, "prefix form: (n) step present")
+assert_true(icol2 ~= nil, "prefix form: italic still rendered")
+
+-- No italic when italic param is nil.
+local text3, _, _, icol3 = mode._test.build_control_text(">", "c", "continue", nil)
+assert_eq(icol3, nil, "no italic span when italic=nil")
+assert_true(text3:find("%(c%)ontinue") ~= nil, "no italic: (c)ontinue rendered")
+
+-- keyof ----------------------------------------------------------------------
+
+-- Save any user override then reset to defaults
+config.merge({})
+assert_eq(mode._test.keyof("continue", "c"), "c", "keyof returns default when unset")
+
+-- Disabled action returns nil
+config.merge({ keys = { continue = false } })
+assert_eq(mode._test.keyof("continue", "c"), nil, "keyof returns nil when key=false")
+
+-- User override
+config.merge({ keys = { continue = "g" } })
+assert_eq(mode._test.keyof("continue", "c"), "g", "keyof returns user override")
+
+-- Empty string falls back to default
+config.merge({ keys = { continue = "" } })
+assert_eq(mode._test.keyof("continue", "c"), "c", "keyof falls back on empty string")
+
+-- Reset config to defaults for subsequent render tests
+config.merge({})
+
+-- render_sbar ----------------------------------------------------------------
+
+do
+  local win, buf = make_bar_win()
+  mode._test.set_bars(win, buf, nil, nil)
+  mode._test.render_sbar()
+  local count = vim.api.nvim_buf_line_count(buf)
+  assert_eq(count, 2, "sbar has exactly 2 lines")
+  local sep = line(buf, 0)
+  local content = line(buf, 1)
+  assert_true(#sep > 0, "sbar separator non-empty")
+  assert_true(content:find(" turbo%-debug ") ~= nil, "sbar contains brand text")
+  assert_true(content:find("DEBUG") ~= nil, "sbar contains DEBUG chip")
+  -- sbar has no click zones
+  local zones = mode._test.get_sbar_zones()
+  assert_eq(#zones, 0, "sbar has zero click zones")
+  mode._test.set_bars(nil, nil, nil, nil)
+  cleanup(win)
+end
+
+-- render_cbar ----------------------------------------------------------------
+
+do
+  local win, buf = make_bar_win()
+  vim.api.nvim_win_set_width(win, 120)
+  mode._test.set_bars(nil, nil, win, buf)
+  mode._test.render_cbar()
+  local count = vim.api.nvim_buf_line_count(buf)
+  assert_eq(count, 2, "cbar has exactly 2 lines")
+  local content = line(buf, 0)
+  local sep = line(buf, 1)
+  assert_true(#sep > 0, "cbar separator non-empty")
+  assert_true(content:find(" help ") ~= nil, "cbar contains 'help' text")
+  -- Click zones: at least 5 controls (continue, step_over, step_into, step_out, terminate) + 1 help
+  -- When no session, restart is hidden so zone count = 5 + 1 = 6.
+  local zones = mode._test.get_cbar_zones()
+  assert_true(#zones >= 6, "cbar has at least 6 click zones: " .. #zones)
+  mode._test.set_bars(nil, nil, nil, nil)
+  cleanup(win)
+end
+
+-- overflow fallback: at narrow widths, italic qualifiers are dropped
+do
+  local win, buf = make_bar_win()
+  vim.api.nvim_win_set_width(win, 25)
+  mode._test.set_bars(nil, nil, win, buf)
+  mode._test.render_cbar()
+  local content = line(buf, 0)
+  -- italic qualifiers are "over", "into", "out" — on overflow they're omitted.
+  -- At width 25 with 5+ controls, there's no way they all fit with italics.
+  assert_false(content:find(" over ") or content:find(" into ") or content:find(" out "),
+    "overflow: italic qualifiers absent at narrow width")
+  mode._test.set_bars(nil, nil, nil, nil)
+  cleanup(win)
+end
+
+-- disabling step_over reduces zone count
+do
+  local win, buf = make_bar_win()
+  vim.api.nvim_win_set_width(win, 120)
+  mode._test.set_bars(nil, nil, win, buf)
+  mode._test.render_cbar()
+  local default_count = #mode._test.get_cbar_zones()
+
+  config.merge({ keys = { step_over = false } })
+  mode._test.render_cbar()
+  local disabled_count = #mode._test.get_cbar_zones()
+  assert_eq(disabled_count, default_count - 1, "disabling step_over drops one zone")
+
+  config.merge({}) -- reset
+  mode._test.set_bars(nil, nil, nil, nil)
+  cleanup(win)
+end
+
+print("ALL RENDER BAR TESTS PASSED")