fix(callcenter): with_jsonl_audit return type io::Result → Result<Self, AuditError>

`UnifiedBridge::with_jsonl_audit` (behind the `jsonl` feature) was declared
`-> std::io::Result<Self>` but its body opens the sink via
`JsonlAuditSink::new(base_path.into())?`, which returns
`Result<_, crate::audit_sink::AuditError>`. `AuditError` has no
`impl Into<std::io::Error>`, so the `?` fails to convert — the function
does not compile whenever a consumer enables the `jsonl` feature.

This means the `jsonl` feature is currently unusable: any crate that turns
it on forces `lance-graph-callcenter` to compile `with_jsonl_audit`, which
errors with E0277 (`?couldn't convert the error to std::io::Error`).

Fix: return `Result<Self, crate::audit_sink::AuditError>` — the honest
type the body already produces, and consistent with the sibling builders
`with_audit_chain` / `flush` / `checkpoint` which all return
`Result<_, AuditError>`. The `?` now works without conversion.

No callers exist (grep: only a doc-comment reference in audit_sink/mod.rs),
so this is not a breaking change for any existing code.

Surfaced by AdaWorldAPI/MedCare-rs#154 (medcare_unified_bridge_audited),
which enables `lance-graph-callcenter/jsonl` and hit the broken build.
Per the consumer's commitment #5 (no silent upstream patches), the fix is
filed here as a proper lance-graph PR rather than carried as a local patch.

Verification: `cargo check -p lance-graph-callcenter --features jsonl` — clean.

Generated by [Claude Code](https://claude.ai/code)

Author: claude

crates/lance-graph-callcenter/src/unified_bridge.rs

@@ -312,7 +312,7 @@ impl<B: NamespaceBridge> UnifiedBridge<B> {
         super_domain: SuperDomain,
         salt: u64,
         base_path: impl Into<std::path::PathBuf>,
-    ) -> std::io::Result<Self> {
+    ) -> Result<Self, crate::audit_sink::AuditError> {
         let sink = Arc::new(crate::audit_sink::JsonlAuditSink::new(base_path.into())?);
         Ok(self.with_audit_chain(super_domain, salt, sink))
     }