Add adjustable refill rate to RotatingTokenBucketLimiter

[parsnips]

Summary

• add SetRefillRate and RefillRate to RotatingTokenBucketLimiter
• preserve existing bucket state while updating refill and rotation timing atomically
• add regression coverage for validation, preserved state, faster/slower refill behavior, and updated rotation intervals

Downstream Context

• parsnips/spreader uses this to split one global scheduling rate across a changing number of publishers in a distributed system
• the scheduler keeps token weight fixed at 255 and adjusts only refill rate so counts above 255 still preserve the intended targetRate / publisherCount share
• rebuilding the limiter on each publisher-count change reset bucket state and could reopen already-consumed capacity; in-place refill updates avoid that while keeping the rotating collision-resistant behavior
• spreader is currently using this via a temporary replace to the forked branch and has regression coverage for the consumed-capacity case that motivated the change

Testing

• go test ./...

[parsnips]

The failing format job is not a formatting issue.

The workflow runs on pull_request_target and then does:

- uses: actions/checkout@v4
  with:
    ref: ${{ github.head_ref }}

For this PR, github.head_ref is adjustable-rotating-rate, but the checkout target is webriots/rate, so Actions is trying to fetch webriots/rate:adjustable-rotating-rate instead of parsnips/rate:adjustable-rotating-rate. The run fails during git fetch before gofumpt ever runs.

The branch is already pushed here: parsnips/rate:adjustable-rotating-rate.

A minimal fix is to check out the PR head repo explicitly, e.g.:

- uses: actions/checkout@v4
  with:
    repository: ${{ github.event.pull_request.head.repo.full_name }}
    ref: ${{ github.event.pull_request.head.ref }}

I verified locally that go mod tidy, gofumpt -l -w ., and go test ./... all pass on this branch.