Fix searcher split cache size boundary

[rdettai-sk]

Description

We have had some issues where the searchers where concurrently downloading a couple of very big splits, exceeding the stores size limit by a lot. This PR prevents this by counting the currently downloaded split sizes against the quota.

This PR also requires indexers to be rolled to be fully operational as those also send notifications about downloadable splits to searchers.

How was this PR tested?

Only unit tests.

[Copilot]

Pull request overview

This PR tightens the searcher split cache quota enforcement by accounting for bytes of in-progress split downloads (not just bytes already present on disk), preventing multiple concurrent large downloads from overshooting the configured cache size limit.

Changes:

• Track and enforce cache byte limits using both on_disk_bytes and newly introduced downloading_bytes in the split cache table.
• Plumb split file sizes (num_bytes) through split reporting (proto + uploader) and through the search path so the cache can reserve/attribute download sizes.
• Make the storage cache wrapper split-scoped (per split id) so it can reliably associate num_bytes with the accessed split.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
quickwit/quickwit-storage/src/split_cache/split_table.rs	Adds download-size tracking (downloading_bytes) and includes downloading splits in limit checks; updates APIs to carry split size.
quickwit/quickwit-storage/src/split_cache/mod.rs	Updates cache wrapping to be split-scoped and threads split id + size into cache lookups and split table updates.
quickwit/quickwit-storage/src/split_cache/download_task.rs	Adjusts CandidateSplit destructuring for the new num_bytes field.
quickwit/quickwit-search/src/leaf.rs	Passes split id + computed split size into split-cache storage wrapping.
quickwit/quickwit-proto/src/codegen/quickwit/quickwit.search.rs	Adds num_bytes to ReportSplit (generated code).
quickwit/quickwit-proto/protos/quickwit/search.proto	Extends ReportSplit message with num_bytes field.
quickwit/quickwit-indexing/src/actors/uploader.rs	Populates ReportSplit.num_bytes from split footer offsets.

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