Un-analyzable files should not pass silently or crash the run #551

Follow-up to #550. While testing rc.25 I hit two related cases where cpp-linter can't actually analyze a file:

1. clang-format exits non-zero (bad config). Fixed the destructive part in #550: an invalid --style, or a .clang-format using a key the installed clang-format version rejects, now logs a warning and skips the file (before, it diffed the empty stdout and proposed deleting the whole file). But the file now counts as clean — checks-failed stays 0, the pre-commit hook passes, and a PR review can approve. A project that bumps its clang version without updating/verifying .clang-format would get green CI even though clang-format never ran.

2. Non-UTF-8 source file. run_clang_format reads the source with fs::read_to_string, so a single latin-1/GBK file (still common in older C/C++) returns stream did not contain valid UTF-8 and aborts the whole run (exit 1) — every other file loses its feedback. (The clang-tidy path reads bytes, so it's unaffected.)

Both come down to one design question: when cpp-linter cannot analyze a file, what should happen?

• Skip + warn (what fix: skip a file when clang-format fails to run #550 does for case 1): safe, never corrupts, but passes silently.
• Surface it as a failure: e.g. count it into checks-failed so the step / pre-commit fails and the output reflects it, while still processing the other files and not generating a patch for the failed one.
• For case 2 specifically: either skip+warn, or diff on raw bytes so non-UTF-8 files can still be formatted. (A lossy from_utf8_lossy isn't safe here — it would corrupt a --fix patch.)

Our lean (Xianpeng and me): case 1 shouldn't pass silently — a broken formatter config ought to fail CI. For case 2 I'd start by just fixing the crash (don't abort the whole run) and handle it the same way as case 1 for consistency; byte-accurate diffing of non-UTF-8 files is a bigger change in the shared diff code and could follow later if there's demand.

@2bndy5 which surface do you prefer? Happy to send a PR once the direction is settled.