wip: feat(userspace/falco): clarify rules validation errors

[AkashKumar7902]

What type of PR is this?

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/kind bug

/kind cleanup

/kind design

/kind documentation

/kind failing-test

/kind feature

/kind release

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/kind chart-release

Any specific area of the project related to this PR?

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/area build

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What this PR does / why we need it:

When --validate receives a top-level YAML mapping such as falco.yaml, rules validation currently reports only a structural error. This change adds an actionable hint explaining that --validate accepts rules files and that --dry-run validates a Falco configuration together with its configured rules.

The hint is added in the validate_rules_files application action, so normal rule loading and embedded engine callers are unchanged. JSON stdout remains machine-parseable; the hint stays in the fatal error summary written to stderr.

Which issue(s) this PR fixes:

Related to #3725

Special notes for your reviewer:

This is a draft because #3725 is assigned to @leogr; I would appreciate confirmation that the diagnostic wording and app-layer placement match the intended scope.

Validation:

• Repository clang-format 18.1.8 and cmake-format 0.6.13 hooks
• CI-matching minimal Linux build of falco_unit_tests
• All three focused ActionValidateRulesFiles regressions
• Full unit suite: 214/214 tests passed
• Manual CLI E2E: baseline returned the generic error; the patch retained exit code 1 and added the --dry-run hint to stdout and stderr
• Text, JSON-preservation, and non-config negative cases

Does this PR introduce a user-facing change?:

feat(userspace/falco): suggest `--dry-run` when `--validate` receives a Falco configuration file

[poiana]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: AkashKumar7902
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[poiana]

Welcome @AkashKumar7902! It looks like this is your first PR to falcosecurity/falco 🎉