feat: capture BigQuery query stats for mktplace metering

[gabrielbressan-tfy]

Summary

• Adds query_stats_registry.py: thread-safe in-process registry accumulating QueryJob stats (total_bytes_billed, cache_hit, project_id) keyed by a mktplace_query_id correlation id passed via ctx.params
• Modifies BigQueryHandler.native_query() to capture stats into the registry after each query execution (via new _record_query_stats helper)
• Adds GET /api/sql/query_stats/<query_id> endpoint that pops and returns the stats JSON; returns {} for unknown ids

Why

mktplace needs to meter actual BigQuery bytes scanned for public datasources (Talentify pays GCP on-demand). The QueryJob.total_bytes_billed was previously discarded immediately. This PR propagates it back to the mktplace caller without touching the result DataFrame pipeline or DuckDB path.

Depends on

Requires the companion PR in Talentify/mktplace (feat/bigquery-usage-metering) which generates the mktplace_query_id, calls query_with_id, and records metering. Deploy this PR first (mindsdb hot-reloads on file change).

Test plan

• Registry unit: accumulate / pop / TTL eviction
• Handler: mock QueryJob with total_bytes_billed=1234567, cache_hit=False; verify registry entry; mock cache_hit=True; verify total_bytes_billed=0
• Live (mindsdb SDK): POST /api/sql/query with params={"mktplace_query_id":"<uuid>"}; GET /api/sql/query_stats/<uuid> → non-zero bytes; repeat (cache hit) → bytes=0
• Non-BQ query: GET /api/sql/query_stats/<unused-uuid> → {}

🤖 Generated with Claude Code

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