#8180)

## Problem / Task
The `internal/memorycache.Cache.startCleanupTimer` exits after one
cleanup due to an unconditional `break` inside the for/select loop. This
causes `BucketStorage` entries used by `SessionLimiter.limitDRL` to
accumulate indefinitely, leading to a massive memory leak (e.g., 1.9 GB
live heap) when using rate limiting by client IP.

## Who is affected?
Users are potentially affected by this memory leak if they meet **all**
of the following conditions:
- Running Tyk Gateway version **5.8.x** (or any version where
`internal/memorycache` was introduced/embedded).
- Using **Rate Limiting** (specifically the default distributed rate
limiter `limitDRL`, which relies on `BucketStorage` backed by
`memorycache`).
- Experiencing a high cardinality of rate limit keys (e.g., rate
limiting by **Client IP** or using custom gRPC plugins that generate
many unique rate limit keys).

## Changes
- Removed the unconditional `break` statement in
`internal/memorycache/cache.go` inside `startCleanupTimer` so the loop
continues to run and clean up expired items.
- Added `TestCache_CleanupTimer` in `internal/memorycache/cache_test.go`
to verify the cleanup timer runs multiple times and correctly removes
expired items.
- Added `BenchmarkCache_MemoryLeak` in
`internal/memorycache/cache_test.go` to validate memory usage and
performance of the cache over time, simulating the memory leak scenario.

## Benchmark Results
To validate the fix, we ran `BenchmarkCache_MemoryLeak` which reports an
`items_left` metric showing how many items remain in the cache at the
end of the run.

**Before the fix (with the unconditional break):**
```text
BenchmarkCache_MemoryLeak-16    	 3703683	      1384 ns/op	    456112 items_left	     101 B/op	       3 allocs/op
```
*Result: 456,112 items left in the cache. The cleanup goroutine was
dead, causing items to accumulate indefinitely.*

**After the fix (unconditional break removed):**
```text
BenchmarkCache_MemoryLeak-16    	 3742993	      1387 ns/op	         1.000 items_left	      90 B/op	       3 allocs/op
```
*Result: Only 1 item left in the cache. The cleanup timer runs
continuously and successfully evicts expired items.*

## Testing
- Ran `go test -v -run TestCache_CleanupTimer ./internal/memorycache`
which passes.
- Ran `go test -bench=BenchmarkCache_MemoryLeak -benchmem -run=^$
./internal/memorycache` which passes and shows stable memory usage.
- Verified the build passes with `make build`.

---------

Co-authored-by: Leonid Bugaev <[email protected]>
Co-authored-by: Matias <[email protected]>
(cherry picked from commit 114bc3c)