[good-first-issue] storage: convert PD backend logging to %-format

Nobody has claimed this yet.

Follow-up for #5118 after #5125.

#5125 enabled Ruff `G004` repo-wide and added temporary `per-file-ignores` for the files/directories that still build logging messages with f-strings. This issue is one small slice of that cleanup queue.

Claiming: please comment `/claim` and wait for assignment so we avoid duplicate PRs.

## Scope

Convert only the logging f-strings in:

- `lmcache/v1/storage_backend/pd_backend.py` (6 G004 findings)
- `lmcache/v1/storage_backend/pd_backend_async.py` (3 G004 findings)

Counts above were measured on `dev` at `dd5dfca` with `ruff check --isolated --select G004`.

## Expected change

- Replace logging f-strings with lazy `%`-style logging arguments, for example `logger.info("loaded %s tokens", num_tokens)`.
- Preserve the rendered log text and behavior.
- Do not change non-logging f-strings or unrelated control flow.
- Remove the matching `G004` ignore entry/entries from `pyproject.toml` once the listed paths are clean.

Matching ignore entry/entries:

- `"lmcache/v1/storage_backend/pd_backend.py" = ["G004"]`
- `"lmcache/v1/storage_backend/pd_backend_async.py" = ["G004"]`

For a `**` directory ignore, delete the directory entry only when the whole listed directory is clean. If a merge conflict leaves other unmigrated files in that ignored directory, replace the broad `**` entry with explicit file-level entries for the leftovers.

## Verification

```bash
ruff check --isolated --select G004 lmcache/v1/storage_backend/pd_backend.py lmcache/v1/storage_backend/pd_backend_async.py
ruff check --select G004 lmcache/v1/storage_backend/pd_backend.py lmcache/v1/storage_backend/pd_backend_async.py pyproject.toml
pre-commit run --all-files
```

The first command should report zero findings after the cleanup. The second command verifies the repo config still passes after removing the ignore entry/entries.

Refs #5118
Refs #3372
Refs #5125

Start with the six G004 findings in lmcache/v1/storage_backend/pd_backend.py and the three findings in lmcache/v1/storage_backend/pd_backend_async.py, then inspect their matching G004 entries in pyproject.toml. Convert only logging f-strings to lazy %-style arguments while preserving rendered text. Run the two specified Ruff checks and pre-commit; done means zero G004 findings and passing checks.

Written by the indexing model from the issue text.