fix(events): stop event run crashing on every piped stdin payload

[Noor-ul-ain001]

Summary

• event_run (src/specify_cli/commands/event.py) capped its stdin read at 1 MiB to prevent a DoS (fix: cap stdin read at 1 MiB to prevent DoS #3857), but the truncation check reads sys.stdin.eof — that attribute does not exist on any Python file-like object, including sys.stdin (hasattr(sys.stdin, "eof") is False).
• Every piped-stdin invocation raised AttributeError: '...' object has no attribute 'eof' instead of running. Piped stdin is this command's documented primary use case ("Resolve and run an event-driven command script with stdin payload" — a native hook feeds it a JSON payload this way), and isatty() is False whenever stdin isn't an interactive terminal, so this fired on essentially every real invocation, not only oversized ones — the fix: cap stdin read at 1 MiB to prevent DoS #3857 DoS fix left the feature entirely broken.
• The intended oversized-payload branch was also broken a second, independent way: raise typer.Exit(code=1, message="...") — typer.Exit.__init__ accepts only code (verified via inspect.signature), not message — so that path raised TypeError instead of the documented clean error.
• Fix: detect truncation the standard way — after reading the 1 MiB cap, read one more byte; a non-empty result means more data was waiting beyond it. Report the oversized-payload error via typer.echo(..., err=True) before raise typer.Exit(code=1).

Test plan

• Added tests/test_event_command.py (no prior test coverage existed for this command): a normal piped payload reaches the handler intact, a TTY/no-stdin invocation falls back to "{}", and an oversized piped payload exits 1 with the limit message instead of crashing.
• Verified all 3 tests fail without the fix — reproduced the exact AttributeError('...' object has no attribute 'eof') on every case, including the "no stdin" one (confirms isatty() is False under non-interactive invocation, matching real hook usage) — and pass with it.
• Ran the new test module standalone — 3 passed.

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01FW9fAYsCBCAgdKWovtSyqt