lint: enforce G004 (lazy %-format logging) with temporary ignores for un-migrated code (#5118) - #5125

What this PR does / why we need it:
Refs #5118
Refs #3372

Enables ruff's G004 ("logging statement uses f-string") and adds temporary
per-file-ignores for the files that have not been migrated to lazy %-format
logging yet — the first step requested by @maobaolong in #5118.

logging renders a record lazily as msg % args, so an f-string pays for its
formatting even when the level is disabled. Today nothing prevents a new
logger.info(f"...") from landing in a file that was already migrated, which is
why this keeps generating one-file good-first-issue PRs: 15+ such PRs have been
merged since March 2026 (#5014, #5094, #4970, #4785, #4727, #4650, #4479, #4357,
#4290, #4142, #4124, #4102, #3977, #3975, #3973) and #4937, #4506, #5106, #5110,
#5112 are still open.

Why the ignore list is mostly per file

Ruff matches per-file-ignores patterns with * crossing directory
separators, so "lmcache/*.py" = ["G004"] silently ignores the whole package
subtree. A naive per-directory list (lmcache/*.py, lmcache/v1/*.py,
lmcache/v1/storage_backend/*.py, ...) would additionally ignore 771
already-migrated files, leaving only 494 of 1337 lintable files protected —
including lmcache/v1/multiprocess/ and lmcache/v1/distributed/, the two
directories that already enforce SLF.

So the list is derived from the actual violations:

• "<dir>/**" = ["G004"] only where every file below that directory violates
  the rule — 4 entries: benchmarks/storage_backend_io, examples/disagg_prefill,
  examples/disagg_prefill_mp, examples/token_dropping/rkv_variants;
• an explicit file entry otherwise — 66 entries.

Result: exactly the 72 violating files are ignored and no clean file is (checked
programmatically), while the rule guards the other 1265 files. Entries can be
deleted one by one as migration continues; each one is either a single file or a
directory that is entirely un-migrated.

Notebooks are covered too — examples/token_dropping/*.ipynb and
examples/token_dropping/rkv_variants/** account for 41 of the 223 violations.

Verification

• ruff check . → All checks passed!, ruff check --statistics . → no findings.
• Enforcement sentinel (three temporary files containing logger.info(f"..."),
  deleted again afterwards): lmcache/v1/multiprocess/<file>,
  lmcache/v1/storage_backend/plugins/<file> (a subdirectory of an un-migrated
  directory) and lmcache/v1/distributed/<file> were all reported as G004,
  while the ignored files stayed quiet.
• Ignore-set audit: the 70 entries cover all 72 violating files and ignore zero
  clean files.
• SKIP=rust-fmt,rust-clippy pre-commit run --all-files → all hooks pass
  (cargo is not installed in my environment).

Special notes for your reviewers:

• This PR only changes pyproject.toml; no runtime code is touched.
• G201 (24 sites) and G002 (1 site) are deliberately not enabled: they change
  what is logged rather than when it is formatted, so they deserve a separate
  decision (noted in [Proposal] Enforce lazy %-format logging (ruff G004) so the f-string logging migration stops regressing #5118).
• The follow-up — delete one ignore entry per migrated file/directory — is a good
  fit for new contributors, as you suggested.

If applicable:

• this PR contains user facing changes - docs added
• this PR contains unit tests