VAULT-21710 - prevent duplicate audit file_path targets

[tvo0813]

Description

Prevent the audit process from allowing duplicate file path targets

TODO only if you're a HashiCorp employee

• Backport Labels: If this PR is in the ENT repo and needs to be backported, backport
  to N, N-1, and N-2, using the backport/ent/x.x.x+ent labels. If this PR is in the CE repo, you should only backport to N, using the backport/x.x.x label, not the enterprise labels.
  • If this fixes a critical security vulnerability or severity 1 bug, it will also need to be backported to the current LTS versions of Vault. To ensure this, use all available enterprise labels.
• ENT Breakage: If this PR either 1) removes a public function OR 2) changes the signature
  of a public function, even if that change is in a CE file, double check that
  applying the patch for this PR to the ENT repo and running tests doesn't
  break any tests. Sometimes ENT only tests rely on public functions in CE
  files.
• Jira: If this change has an associated Jira, it's referenced either
  in the PR description, commit message, or branch name.
• RFC: If this change has an associated RFC, please link it in the description.
• ENT PR: If this change has an associated ENT PR, please link it in the
  description. Also, make sure the changelog is in this PR, not in your ENT PR.

[github-actions]

CI Results:
All Go tests succeeded! ✅

[github-actions]

Build Results:
All builds succeeded! ✅

[ltcarbonell]

Could you add a sample of the new error response from Vault to the description? This will help us see how Vault responds without needing to run it manually.

[tvo0813]

Tested locally and is working as it should (failed when enabling same file_path)

tin.vo@tin command % vault audit enable -path=file1 file file_path=${APATH};             
Success! Enabled the file audit device at: file1/
tin.vo@tin command % vault audit enable -path=file2 file log_raw=true file_path=${APATH};
Error enabling audit device: Error making API request.

URL: PUT http://127.0.0.1:8200/v1/sys/audit/file2
Code: 400. Errors:

* file_path already in use: invalid configuration

2024-10-23T09:25:10.924-0700 [INFO]  core: enabled audit backend: path=file1/ type=file
2024-10-23T09:25:15.533-0700 [ERROR] core: enable audit mount failed: path=file2/ error="file_path already in use: invalid configuration"```

[kubawi]

LGTM, thanks Tin!

I am leaving an approval and some suggestions to make comments clearer.