Fix error handling from BeatV2Manager reload

[belimawr]

Proposed commit message

BeatV2Manager `reload` method calls `reloadInputs`, however it did not
handle the errors returned correctly. This commit fixes it by making
`reloadInputs`, whenever possible, use a cfgfile.UnitError to indicate
the failed unit and `reload` now handles any unexpected error case/type.

Multiple lint warnings are fixed, including:
 - An unused wait group is removed
 - The parameters order in a `errors.Is` call is fixed

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

## Disruptive User Impact
## Author's Checklist

How to test this PR locally

Related issues

• Closes Beats silently fails to start any input if there is an error when parsing the units sent by Elastic-Agent (emtpy inputs in beat-rendered-config.yaml) #43118

## Use cases
## Screenshots
## Logs

[github-actions]

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[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @belimawr? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[Copilot]

Pull Request Overview

This PR fixes error handling in the BeatV2Manager's reload functionality by improving how errors from the reloadInputs method are processed and reported. The fix ensures that unit-specific errors are properly wrapped using cfgfile.UnitError and that unexpected error types are handled gracefully.

• Enhanced error handling in the reload method to properly categorize and handle different error types
• Modified reloadInputs to return cfgfile.UnitError for unit-specific failures instead of generic errors
• Added comprehensive test coverage for error handling scenarios

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
x-pack/libbeat/management/managerV2.go	Core error handling improvements with proper error categorization and unit-specific error wrapping
x-pack/filebeat/tests/integration/managerV2_test.go	New integration test for reload error handling scenarios and type updates
x-pack/libbeat/management/simple_input_config_test.go	Type updates from interface{} to any and minor formatting changes

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[github-actions]

@Mergifyio backport 8.17 8.18 8.19 9.0 9.1

[mergify]

backport 8.17 8.18 8.19 9.0 9.1

✅ Backports have been created

Details

• #45756 [8.17](backport #45733) Fix error handling from BeatV2Manager reload has been created for branch 8.17 but encountered conflicts
• #45757 [8.18](backport #45733) Fix error handling from BeatV2Manager reload has been created for branch 8.18 but encountered conflicts
• #45758 [8.19](backport #45733) Fix error handling from BeatV2Manager reload has been created for branch 8.19 but encountered conflicts
• #45759 [9.0](backport #45733) Fix error handling from BeatV2Manager reload has been created for branch 9.0
• #45760 [9.1](backport #45733) Fix error handling from BeatV2Manager reload has been created for branch 9.1