Warn when an explicit target_modules entry matches no module

## Proposal

In https://github.com/huggingface/peft/issues/3658#issuecomment-5538703779, @BenjaminBossan suggested warning when an explicit `target_modules` entry matches nothing (for example, `["q_proj", "v_proj", "foobar_proj"]`). The later discussion explicitly leaves this separate from #3658's per-layer coverage reporting.

Would you be happy for me to take that narrower warning as a separate contribution?

Proposed scope:

- For explicit target-name collections, report entries that match no eligible module when other entries do match, without changing which adapters are injected.
- Preserve the existing error when nothing matches.
- Leave regex decomposition, uneven layer coverage, and new status-reporting fields out of scope.
- Add small CPU-only tests for partial matches, complete matches, and the existing no-match error.

Before coding, I'd like to clarify whether entries matching modules that are deliberately excluded (`exclude_modules`, layer selection, or `modules_to_save`) should be omitted from the warning. My preference is to avoid warning for deliberate exclusions and focus on names absent from the model.

I checked current open PRs for `target_modules` and searches for the unmatched-target warning; I did not find an implementation of this separate request. I have not implemented a patch and will wait for approval and ownership confirmation.

AI assistance: Codex helped inspect the current matching code, search for overlap, and prepare this proposal. Any eventual AI-assisted patch will need human review and testing before submission, as required by the repository's contribution policy.

Start by inspecting the current target_modules matching code and how excluded modules are handled. Add small CPU-only tests covering partial matches, complete matches, and the existing no-match error; done means unmatched explicit entries warn without changing adapter injection, while deliberate exclusions do not warn.

Written by the indexing model from the issue text.