fix: exit nonzero when a circuit fails to build correctly

[billythompsons]

Fixes #4707.

Problem

tsci build printed "Build completed with errors" yet exited 0 and counted the circuit as passed when the circuit itself failed to build - e.g. a trace targeting a nonexistent component (source_trace_not_connected_error). CI cannot detect the invalid design from the process status.

Fix

The build now distinguishes an invalid circuit from DRC violations on an otherwise valid one:

• circuit-json-diagnostics gains isCircuitFailureError, currently matching source_trace_not_connected_error: a trace endpoint that resolves to no existing port is an invalid design, never a legitimate partial build.
• buildFile (and the build worker) report hasCircuitErrors when such an error is present (respecting --ignore-errors, after DRC ignore-category filtering).
• tsci build exits 1 when any circuit has a circuit failure (fatal errors still always exit 1), and failed circuits are no longer counted as passed in the summary.

Deliberately scoped narrow: routing failures like pcb_trace_missing_error stay exit 0 because valid simulation/partial circuits can legitimately leave traces unrouted (covered by the existing build-output-flags simulation tests, which pass unchanged). DRC violations (e.g. pcb_component_outside_board_error) also stay exit 0, as do warnings. Happy to extend the failure set in a follow-up if you want routing failures included.

Tests

• New build-exit-code-circuit-errors.test.ts: the issue's exact repro (trace to a nonexistent component) exits 1 with "0 passed / 1 failed"; with --ignore-errors exits 0.
• Existing exit-code/DRC/simulation contract tests pass unchanged: build-with-drc-error, build-with-ignore-error, build-ignore-drc-categories, build-ignore-placement-drc-routes, build-output-flags.

[billythompsons]

Closing - #4715 landed just before this and covers the same ground (exit 1 on circuit errors, artifacts retained, failed circuits not counted as passed). Thanks!