perf(agentos): batch chunked_sqlite index saves into transactions

[eersnington]

• The index is saved after every filesystem change. Each save was 2N + 4 separate SQL requests for N pieces.
• The same statements are now sent in transactions of up to 32 statements. VM SQLite requests are JSON and a sidecar frame is at most 16 MiB, so each request stays under about 8 MiB. An index under 2 MiB (about 6,000 files) is saved in one transaction.
• The head update and one DELETE of the other generations go in the last transaction. A load reads the previous index until that transaction commits.
• Adds a test: after a save whose reply is lost and a save that fails, the index still loads.

Step	Before	After
Next generation	SELECT MAX(generation)	same
Write pieces	one INSERT request per piece	up to 32 statements per transaction
Move the head	separate UPSERT request	in the last transaction
Remove old pieces	SELECT + one DELETE per piece, after the save	one DELETE in the last transaction

let mut statements = pieces.map(/* INSERT piece */).chain([
    /* UPSERT head -> generation */,
    /* DELETE FROM agentos_fs_metadata_chunks WHERE namespace = ? AND generation <> ? */,
]);
loop {
    let batch = statements
        .by_ref()
        .take(METADATA_SAVE_STATEMENTS_PER_TRANSACTION)
        .collect::<Vec<_>>();
    if batch.is_empty() {
        return Ok(());
    }
    database.transaction(batch).await.map_err(actor_sql_error)?;
}

Code: crates/native-sidecar/src/plugins/chunked_sqlite.rs

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This is part 3 of 4 in a stack:

•  4  fix(agentos): route CompareVmConfig through the sidecar request loop #2018
•  3  perf(agentos): batch chunked_sqlite index saves into transactions #2017 👈
•  2  fix(agentos): keep chunked_sqlite metadata pieces under the remote SQLite bind limit #2016
•  1  fix(agentos): accept [ as a package command name #2015

[the-company-company]

🔴 1 high-severity finding

_{Reviewed commit 5d11641.}

[the-company-company]

🔴 1 high-severity finding

_{Reviewed commit 2aac4f9.}

[the-company-company]

✅ No issues found

_{Reviewed commit 9f59435.}