fix: compactor file descriptor leak

[jpinsonneau]

What this PR does / why we need it:

Fixes a file descriptor leak in the TSDB compactor's setupBuilder function that occurs when processing many source index files. The leak causes file descriptors to accumulate during compaction, eventually leading to resource exhaustion and "no space left on device" errors.

Which issue(s) this PR fixes:
Fixes #19514

Special notes for your reviewer:

Created a dedicated test case for that issue.

• Before fix:

go test -v ./pkg/storage/stores/shipper/indexshipper/tsdb -run TestSetupBuilder_FileDescriptorLeak
=== RUN   TestSetupBuilder_FileDescriptorLeak
    compactor_fd_leak_test.go:148: File descriptor stats: initial=8, peak=58, avg=35, final=8, increase=50, numFiles=50, samples=2
    compactor_fd_leak_test.go:164: File descriptor leak detected! Peak increase: 50 (expected leak: ~50, allowed: 20). This indicates files are not being closed immediately during processing. With the fix, file descriptors should be closed right after use, not deferred until function return. FD samples show accumulation pattern.
--- FAIL: TestSetupBuilder_FileDescriptorLeak (0.24s)
FAIL
FAIL	github.com/grafana/loki/v3/pkg/storage/stores/shipper/indexshipper/tsdb	0.264s
FAIL

• After fix:

go test -v ./pkg/storage/stores/shipper/indexshipper/tsdb -run TestSetupBuilder_FileDescriptorLeak
=== RUN   TestSetupBuilder_FileDescriptorLeak
    compactor_fd_leak_test.go:148: File descriptor stats: initial=8, peak=11, avg=10, final=8, increase=3, numFiles=50, samples=2
--- PASS: TestSetupBuilder_FileDescriptorLeak (0.48s)
PASS
ok  	github.com/grafana/loki/v3/pkg/storage/stores/shipper/indexshipper/tsdb	0.518s

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
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