fix(tsc): guard "No errors found" behind exit code

[NekomyaDev]

Closes #3220. (Re-targeted to develop per CONTRIBUTING.md — supersedes #3294.)

Problem

TscHandler::format_summary ignored the process exit code: whenever zero parseable TS#### errors were captured, it returned TypeScript: No errors found — even when tsc exited non-zero without emitting parseable diagnostics (e.g. missing tsconfig.json, invalid compiler flags, fatal config errors like TS5058). The filter could therefore report success on a failed run.

Fix

format_summary now guards on exit_code:

• exit_code == 0 + no parseable errors → unchanged: TypeScript: No errors found
• exit_code != 0 + no parseable errors → reports TypeScript: failed (exit code N) with no parseable TS errors and appends the last 10 non-empty lines of raw output so the fatal diagnostic (which the streaming filter otherwise drops) stays visible

Tests

Two new regression tests:

• test_tsc_stream_failure_without_parseable_errors — non-zero exit with only a fatal TS5058 line must not contain "No errors found", must surface exit code and the fatal line
• test_tsc_stream_success_still_reports_no_errors — zero exit with empty output still reports "No errors found"

All existing tsc streaming/filter tests are unaffected (the success path is unchanged).