chore: converge stale TODOs and apply safe fills (backlog#646) (#4322)

Second TODO-convergence round over the current tree (backlog#646). All
line numbers in the old inventory had gone stale after the set_disk /
diagnostics / cluster refactors, so this re-scans and reduces the marker
count from 144 to 99.

STALE removals (comment describes already-implemented behavior, or dead
commented-out blocks) across ecstore (set_disk ops/core, store,
cluster/rpc, bucket/metadata_sys, services), iam, filemeta, s3select and
rustfs auth/object_usecase. No behavior change.

Safe fills, each verified:
- filemeta: replication_info_equals now also compares
  replication_state_internal (function currently has no callers; adds a
  regression test).
- bitrot: drop the confirmed-unused `_want` parameter from bitrot_verify
  and the now-unused `sum` on LocalDisk::bitrot_verify, removing a
  Bytes::copy_from_slice allocation. Streaming verify uses the file's
  embedded per-shard hash, never the passed sum.
- signer: rename v4_ignored_headers -> V4_IGNORED_HEADERS and drop the
  non_upper_case_globals allow.
- admin/heal: test_decode was #[ignore]d and used serde_urlencoded on a
  JSON body (would panic); rewire to serde_json::from_slice to match the
  production decode path, add assertions, un-ignore.

Verified: cargo fmt; cargo check on touched crates; tests pass
(filemeta, signer, bitrot, heal::test_decode); arch guardrail scripts
pass.

crates/ecstore/src/object_api/types.rs

@@ -365,7 +365,6 @@ impl ObjectInfo {
             if let Some(size_str) = rustfs_utils::http::get_str(&self.user_defined, rustfs_utils::http::SUFFIX_ACTUAL_SIZE)
                 && !size_str.is_empty()
             {
-                // Todo: deal with error
                 let size = size_str.parse::<i64>().map_err(|e| std::io::Error::other(e.to_string()))?;
                 return Ok(size);
             }
@@ -509,8 +508,6 @@ impl ObjectInfo {
             })
             .collect::<Vec<_>>();
 
-        // TODO: part checksums
-
         ObjectInfo {
             bucket: bucket.to_string(),
             name,