fix(settings): validate DUCKDB_THREADS instead of silently producing NaN

[pedrofrxncx]

Bug (same class as PORT/DATABASE_POOL_MAX, this tick's other session): resolveConfig parsed DUCKDB_THREADS as envVars.DUCKDB_THREADS ? Number(envVars.DUCKDB_THREADS) : undefined — unlike CLICKHOUSE_MAX_MEMORY_USAGE (which falls back to undefined via || undefined), an invalid value here (e.g. DUCKDB_THREADS=abc) produces NaN, which is then threaded into DuckDBTuning.threads and stringified into the DuckDB instance threads config (apps/mesh/src/monitoring/query-engine.ts), causing a cryptic runtime failure when the embedded monitoring DuckDB engine spins up instead of a clear startup error.

A maintainer wants this because it turns a silent/cryptic monitoring-engine failure into a clear fail-fast error at config-resolution time, matching the validation already applied to PORT and DATABASE_POOL_MAX.

Fix: added toPositiveIntegerOrUndefined (same shape as the existing toPositiveIntegerOrDefault but with no default, since DuckDB threads legitimately has no numeric default — unset means "use all CPUs") and used it for DUCKDB_THREADS. Throws "DUCKDB_THREADS must be a positive integer" for non-integer/non-positive values, same as the sibling validators.

Regression test: added a describe("resolveConfig duckdb threads") block to resolve-config.test.ts covering the unset-default, valid-value, and invalid-value-throws cases (mirroring the existing DATABASE_POOL_MAX/NATS_TUNNEL_SESSION_TTL_SECONDS test blocks).

Reviewer check: bun test apps/mesh/src/settings/resolve-config.test.ts

Locally ran: bun run fmt, cd apps/mesh && bunx tsc --noEmit, and the targeted test file above (all green). Full CI validates the rest.

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Summary by cubic

Validate DUCKDB_THREADS during config resolution to fail fast on invalid values, preventing NaN from reaching DuckDB and causing a cryptic startup crash in the monitoring engine.

• Bug Fixes
  • Added toPositiveIntegerOrUndefined and used it for DUCKDB_THREADS; invalid values throw "DUCKDB_THREADS must be a positive integer", unset stays undefined (use all CPUs).
  • Added tests for unset, valid, and invalid cases in apps/mesh/src/settings/resolve-config.test.ts.

^{Written for commit 9bd1d79. Summary will update on new commits.}