fix(rivetkit): copy Buffer to Uint8Array in ensureUint8Array to fix .slice() semantics (#4506)

# Description

Please include a summary of the changes and the related issue. Please also include relevant motivation and context.

## Type of change

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
- [ ] This change requires a documentation update

## How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

## Checklist:

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my feature works
- [ ] New and existing unit tests pass locally with my changes

Author: NathanFlurry

rivetkit-typescript/packages/rivetkit/src/drivers/file-system/sqlite-runtime.ts

@@ -216,6 +216,16 @@ export function ensureUint8Array(
 	fieldName: string,
 ): Uint8Array {
 	if (value instanceof Uint8Array) {
+		// Buffer (from better-sqlite3) extends Uint8Array but overrides
+		// .slice() to return a view instead of a copy. This breaks
+		// downstream libraries (vbare, cbor-x) that rely on
+		// Uint8Array.prototype.slice() semantics. Copy into a plain
+		// Uint8Array so .slice() always copies.
+		if (typeof Buffer !== "undefined" && Buffer.isBuffer(value)) {
+			const copy = new Uint8Array(value.byteLength);
+			copy.set(value);
+			return copy;
+		}
 		return value;
 	}
 	if (value instanceof ArrayBuffer) {