Fix SPARQL parser backtrack, executor memory leak, and add catch_unwind

[grparry]

Summary

Three additional hardening fixes for the SPARQL subsystem, building on PR #172:

• Parser backtrack fix: Replace hardcoded saturating_sub(6) with a saved_pos variable. The old backtrack assumed all update keywords are 6 characters, but LOAD (4), DROP (4), and CLEAR (5) are shorter, causing incorrect parse positions for those statements.

• Executor memory leak fix: Change SparqlContext.default_graph from Option<&'a str> to Option<String> and remove Box::leak calls in the GraphPattern::Graph handler. Each GRAPH clause previously leaked a String allocation that was never freed.

• catch_unwind safety net: Wrap ruvector_sparql() parse/execute/format in std::panic::catch_unwind so that panics from non-empty but malformed queries are converted to PostgreSQL ERROR messages instead of crashing the backend. This complements the empty-query check from fix: HNSW index bugs, agent/SPARQL crashes, lru security #172 by catching deeper failures.

Closes #167

Test plan

• Verify LOAD, CLEAR, DROP SPARQL update statements parse correctly (parser backtrack)
• Verify GRAPH <iri> { ... } queries don't leak memory (executor fix)
• Verify malformed non-empty SPARQL queries return ERROR, not SIGABRT (catch_unwind)
• Verify existing SPARQL tests still pass

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