Add comprehensive tests for diagnostics tools, remove coverage exclusion

Covers all 13 debug tool handlers including set_breakpoints (path
mapping + warnings), get_variables (scope filtering, object_path,
truncation), wait_for_stop (immediate halt, timeout, waiter
resolution), capture_at_breakpoint (full orchestration, trigger_url,
eval errors, timeout), start_session (real fixture cartridge + stubbed
manager), and all step_* variants. Registry tests cover idle cleanup
and disconnect error swallowing.

Coverage is now 99.81% statements/lines, 94.65% branches, 98.94%
functions — meeting all thresholds without excluding diagnostics.

Author: clavery

packages/b2c-dx-mcp/.c8rc.json

@@ -1,7 +1,7 @@
 {
   "all": true,
   "src": ["src"],
-  "exclude": ["test/**", "**/*.d.ts", "**/index.ts", "**/site-theming/types.ts", "**/tools/diagnostics/**"],
+  "exclude": ["test/**", "**/*.d.ts", "**/index.ts", "**/site-theming/types.ts"],
   "reporter": ["text", "text-summary", "html", "lcov"],
   "report-dir": "coverage",
   "check-coverage": true,