Fix crash when buffer is modified before callback fires

[benthamite]

Summary

This is a companion to #39. That PR guarded against the source buffer being killed before the url-retrieve callback fires; this one guards against the buffer being modified (i.e. still alive, but with different contents).

• flycheck-languagetool--check-all: skip matches whose offset + length falls outside the current buffer bounds instead of signaling args-out-of-range
• flycheck-languagetool--read-results: move the condition-case to wrap the entire callback invocation, fixing two issues:
  1. The error handler's return value previously fell through to mapcar, causing "Wrong type argument: sequencep"
  2. callback could be invoked twice (once with 'errored, once with 'finished)

Together with #39, this covers all three async race conditions in the callback: buffer killed, buffer modified, and response parse failure.

Problem

When the source buffer is edited while the LanguageTool HTTP request is in flight, the byte offsets in the JSON response can exceed (point-max). This causes:

Error from syntax checker languagetool: Args out of range: 242, 1, 1
mapcar: Wrong type argument: sequencep, 69

The args-out-of-range error in --check-all is caught by the condition-case error handler, which calls (funcall callback 'errored ...). But because the condition-case was placed inside the mapcar argument, the handler's return value is passed to mapcar as a sequence — triggering the second error. With debug-on-error active, this enters the Emacs debugger in a recursive edit, freezing the UI.

Test plan

Includes 9 ERT tests covering:

• Valid matches within buffer bounds are returned correctly
• Matches with offset past point-max are silently skipped
• Matches with offset + length past point-max are silently skipped
• Mix of valid and stale matches: only valid ones returned
• Empty matches list returns nil
• Multiline buffer positions produce correct line numbers
• Dead source buffer → callback called once with 'interrupted
• Malformed response → callback called exactly once with 'errored
• Well-formed response → callback called once with 'finished

[mavit]

Thanks.

Could you split the flycheck-languagetool--read-results fix into a separate commit to the flycheck-languagetool--check-all changes?

Could you plumb the tests into the Eask config? Is it feasible to run them as GitHub CI tests?

[benthamite]

Yes, done.