fix(cache): allow --help flag for cache subcommand

[NORMAL-EX]

Summary

Fixes #3560

Problem

bat cache --help currently fails with:

error: unexpected argument '--help' found

Usage: bat cache [OPTIONS] <--build|--clear>

This happens because the cache-actions ArgGroup is set to required(true), which causes clap to validate required arguments before processing the --help flag.

Solution

Remove .required(true) from the cache-actions ArgGroup. The behavior remains the same (users must specify either --build or --clear when running the cache subcommand) because clap will still show an error if neither is provided. However, now --help is processed first, allowing users to view the help message.

Testing

# Before: error
$ bat cache --help
error: unexpected argument '--help' found

# After: shows help
$ bat cache --help
Modify the syntax-definition and theme cache
...

---

This is a resubmission of #3573 with cargo fmt applied.

[NORMAL-EX]

The cargo audit failure is unrelated to this PR - it's caused by a newly published security advisory RUSTSEC-2026-0009 (published 2026-02-05) affecting the time crate dependency.

This affects all PRs until the upstream dependency is updated in the main branch.

[keith-hall]

Looks like the cache_help_shows_help_message integration test from the original PR got omitted?

[keith-hall]

erm, the changelog entry being repeated for every version of bat ever released is a bit... odd... we can't go back in time / backport the fix and republish old versions with the same version number it originally had... I suggest just to keep the entry in the unreleased section...

[NORMAL-EX]

Fixed both issues:

1. Reverted CHANGELOG.md to only have the entry in the unreleased section — sorry about that!
2. Added the cache_help_shows_help_message integration test from the original PR

Thanks for catching these!

[NORMAL-EX]

Updated the fix to match the approach from #3573:

• Added .arg_required_else_help(true) to the cache subcommand
• Replaced the ArgGroup with .conflicts_with() on each arg
• Removed unused ArgGroup import

Just removing .required(true) from the group wasn't enough — --help was still rejected. The full removal of the ArgGroup + arg_required_else_help is needed for clap to properly handle --help.

[NORMAL-EX]

Found the root cause of the test failure: the parent command sets .disable_help_flag(true) (line 37), which propagates to the cache subcommand. This means --help is not recognized on bat cache regardless of the ArgGroup changes.

Added .disable_help_flag(false) to the cache subcommand to override the parent setting. Also fixed cargo fmt (import line formatting).