Report a nonexistent decode input the same way every other command does

[magic-peach]

The decode command opened its input file with a raw std::ifstream guarded only by assert(!input_stream.fail()), the only command in the CLI still doing this instead of the shared read_file helper every other command (encode and 7 others) already uses.

In a build where assertions are compiled in, a nonexistent .binpack path crashes the process outright: Assertion failed: (!input_stream.fail()), function decode, file command_decode.cc, line 61, exit code 134. Confirmed this directly by reverting just the fix and running decode against a missing file.

This switches to read_file, which routes through the same IOFileNotFoundError handling every other command already relies on, turning the crash into the standard error: No such file or directory that decode, validate, and the rest of the CLI already give for this class of error.

Verified by adding test/decode/fail_enoent.clitest, matching the existing test/validate/fail_schema_enoent.clitest convention, and running the full test suite locally: 1136 tests, all passing.

[augmentcode]

🤖 Augment PR Summary

Summary: This PR standardizes decode input-file handling with the rest of the CLI.

Changes:

• Replaces manual binary `ifstream` opening and assertions with `sourcemeta::core::read_file`.
• Routes nonexistent decode inputs through the shared `IOFileNotFoundError` handling path instead of aborting when assertions are enabled.
• Adds plain-text and `--json` CLI regression coverage for a missing `.binpack` input.
• Registers the new decode failure test in the CMake test suite.

Technical Notes: The shared helper preserves binary stream semantics and reports canonical file-path diagnostics consistently with other commands.

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[augmentcode]

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[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}

[jviotti]

Good catch!