fix(unit tests): Panic caused by edge case during unit test build

[Kalopsian-tz]

Summary

I was working on the unit test included in the config documented below which is invalid because filter_msg is listed in both no_outputs_from and outputs.extract_from.

The issue arises when you try to run the invalid test, instead of handling the issue cleanly, Vector panics instead:

$ vector test vector.yaml
thread 'main' panicked at src/config/unit_test/mod.rs:75:49:
An unexpected error occurred while executing unit tests. Please try again.: RecvError(())

This seems to happen because Vector doesn't include a check that output.extract_from entries are not also listed in no_outputs_from which allows it to pass the build phase and fail when the invalid test is unwrapped, causing the panic.

This PR adds a check which resolves the issue and lets Vector exit cleanly with a defined error that follows the format used by another error in the same function.

$ vector test vector.yaml
Running tests
2025-08-22T01:59:08.590667Z ERROR vector::unit_test: Failed to execute tests:
Failed to build test 'Invalid test':
  Invalid extract_from target in test 'Invalid test': 'filter_msg' listed in no_outputs_from.

PS: This is my first time raising a PR on an open source project so any feedback on how I raised / worded this PR would be greatly appreciated! (I hope its ok I made a PR without making an issue first? The contributing page said "For large PRs or for breaking changes, ensure your change has an issue!" I couldn't find another issue raised about this and figured this wasn't a large or breaking change)

Vector configuration

sources:
  cmd:
    type: exec
    command:
      - echo
      - "a message!"
    mode: scheduled
    scheduled:
      exec_interval_secs: 120

transforms:
  filter_msg:
    type: filter
    inputs:
      - cmd
    condition: |
      .message != "a message!"

tests:
- name: Invalid test
  no_outputs_from:
    - filter_msg
  inputs:
  - insert_at: filter_msg
    type: log
    log_fields:
      message: "a message!"
  outputs:
  - extract_from: filter_msg
    conditions:
    - type: vrl
      source: |
        assert_eq!(.message, "a message!")

sinks:
  console:
    type: console
    encoding:
      codec: json
    inputs:
      - filter_msg

How did you test this PR?

I trialed similar edge cases I could think of to ensure they also pass with this new check as well as running the suggested tests:

cargo fmt --all
cargo clippy --workspace --all-targets -- -D warnings

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

Notes

• Please read our Vector contributor resources.
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  • Alternatively, we recommend running the following locally before pushing to the remote branch:
    • cargo fmt --all
    • cargo clippy --workspace --all-targets -- -D warnings
    • cargo nextest run --workspace (alternatively, you can run cargo test --all)
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  • For example, you can run git merge origin master and git push.
• If this PR introduces changes Vector dependencies (modifies Cargo.lock), please
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