[BUG](sysdb): Take write lock in delete_collection

[vishnujayvel]

SqliteSysDb::delete_collection opens its transaction with begin(), which starts a deferred SQLite transaction — the write lock isn't taken until the first actual write. In that window another connection can open its own transaction on the same collection name and interleave, so a concurrent create/delete pair can both report success and lose an update. create_collection already guards against this by following begin() with begin_immediate(); delete_collection did not. Credit to @roli-lpci for reporting #7375 and pinning down that asymmetry — the diagnosis in the issue is exactly right.

This adds the same begin_immediate() call so the write lock is acquired up front and the two paths behave consistently.

let mut tx = self.db.get_conn().begin().await?;
// now also in delete_collection, matching create_collection:
self.db.begin_immediate(&mut *tx).await?;

Verified with cargo test -p chroma-sysdb sqlite:: (15 passed), cargo fmt --check, and cargo clippy -p chroma-sysdb --lib -- -D warnings, on a branch merged with current main.

Closes #7375

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