perf: Fix memory leak in cachedIterator

[benclive]

What this PR does / why we need it:
Fixes closing the wrapper iterator in cachedIterators when Close is called.

• I was looking into the lz4-based memory leak reported in an issue, and noticed we only leak memory when running FORWARD queries and not BACKWARD queries.
• This cached iterator is often only used for FORWARD queries, so we don't see this leak in our day-to-day as we usually only run BACKWARD queries frequently
• I've attached a benchstat after making this change. Mostly unchanged results except much lower bytes allocated on FORWARD queries.

goos: darwin
goarch: arm64
pkg: github.com/grafana/loki/v3/pkg/logql/bench
cpu: Apple M3 Max
                                                                                    │ before_fix.txt │            after_fix.txt            │
                                                                                    │     sec/op     │   sec/op     vs base                │
LogQL/query={region="ap-southeast-1",_env="dev"}_[BACKWARD]/kind=log/store=chunk-14      228.4m ± 1%   235.7m ± 4%   +3.18% (p=0.000 n=10)
LogQL/query={region="ap-southeast-1",_env="dev"}_[FORWARD]/kind=log/store=chunk-14       49.46m ± 1%   34.29m ± 6%  -30.67% (p=0.000 n=10)
geomean                                                                                  106.3m        89.90m       -15.42%

                                                                                    │   before_fix.txt   │                after_fix.txt                │
                                                                                    │ kilobytesProcessed │ kilobytesProcessed  vs base                 │
LogQL/query={region="ap-southeast-1",_env="dev"}_[BACKWARD]/kind=log/store=chunk-14          12.10k ± 0%          12.10k ± 0%       ~ (p=1.000 n=10) ¹
LogQL/query={region="ap-southeast-1",_env="dev"}_[FORWARD]/kind=log/store=chunk-14            362.7 ± 0%           362.7 ± 0%       ~ (p=1.000 n=10) ¹
geomean                                                                                      2.095k               2.095k       +0.00%
¹ all samples are equal

                                                                                    │ before_fix.txt │              after_fix.txt              │
                                                                                    │ linesProcessed │ linesProcessed  vs base                 │
LogQL/query={region="ap-southeast-1",_env="dev"}_[BACKWARD]/kind=log/store=chunk-14      35.59k ± 0%      35.59k ± 0%       ~ (p=1.000 n=10) ¹
LogQL/query={region="ap-southeast-1",_env="dev"}_[FORWARD]/kind=log/store=chunk-14       1.070k ± 0%      1.070k ± 0%       ~ (p=1.000 n=10) ¹
geomean                                                                                  6.171k           6.171k       +0.00%
¹ all samples are equal

                                                                                    │ before_fix.txt  │              after_fix.txt               │
                                                                                    │ postFilterLines │ postFilterLines  vs base                 │
LogQL/query={region="ap-southeast-1",_env="dev"}_[BACKWARD]/kind=log/store=chunk-14       35.59k ± 0%       35.59k ± 0%       ~ (p=1.000 n=10) ¹
LogQL/query={region="ap-southeast-1",_env="dev"}_[FORWARD]/kind=log/store=chunk-14        1.070k ± 0%       1.070k ± 0%       ~ (p=1.000 n=10) ¹
geomean                                                                                   6.171k            6.171k       +0.00%
¹ all samples are equal

                                                                                    │ before_fix.txt │             after_fix.txt             │
                                                                                    │      B/op      │     B/op       vs base                │
LogQL/query={region="ap-southeast-1",_env="dev"}_[BACKWARD]/kind=log/store=chunk-14    218.2Mi ± 10%   214.3Mi ± 10%        ~ (p=1.000 n=10)
LogQL/query={region="ap-southeast-1",_env="dev"}_[FORWARD]/kind=log/store=chunk-14     608.8Mi ±  1%   105.0Mi ±  2%  -82.75% (p=0.000 n=10)
geomean                                                                                364.5Mi         150.0Mi        -58.84%

                                                                                    │ before_fix.txt │           after_fix.txt            │
                                                                                    │   allocs/op    │  allocs/op   vs base               │
LogQL/query={region="ap-southeast-1",_env="dev"}_[BACKWARD]/kind=log/store=chunk-14      725.1k ± 0%   725.1k ± 0%       ~ (p=0.971 n=10)
LogQL/query={region="ap-southeast-1",_env="dev"}_[FORWARD]/kind=log/store=chunk-14       421.8k ± 0%   421.1k ± 0%  -0.16% (p=0.000 n=10)
geomean                                                                                  553.0k        552.6k       -0.08%

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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