fix(settings): validate CLICKHOUSE_MAX_MEMORY_USAGE instead of silently ignoring invalid values

[pedrofrxncx]

Follows #5129 (PORT validation) and the same pattern applied to DUCKDB_THREADS in #5130 — resolve-config.ts has more than one env var that silently swallows a bad value instead of failing loudly.

clickhouseMaxMemoryUsage was computed as Number(envVars.CLICKHOUSE_MAX_MEMORY_USAGE) || undefined. Any non-numeric or invalid value ("abc", "-1", "1.5") silently produces NaN which then falls through || undefined, so a typo in this env var quietly disables the ClickHouse memory-limit engine option in production instead of failing startup — the same failure mode PORT and DUCKDB_THREADS just got fixed for.

Fix: added a toPositiveIntegerOrUndefined helper (same shape as the existing toPositiveIntegerOrDefault used for PORT/DATABASE_POOL_MAX) and use it for CLICKHOUSE_MAX_MEMORY_USAGE. Unset/empty still resolves to undefined (unchanged default behavior — context-factory.ts's settings.clickhouseMaxMemoryUsage ? {...} : ... check is unaffected), but an invalid value now throws "CLICKHOUSE_MAX_MEMORY_USAGE must be a positive integer" at startup instead of being silently dropped.

To verify: bun test apps/mesh/src/settings/resolve-config.test.ts

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

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Summary by cubic

Validate CLICKHOUSE_MAX_MEMORY_USAGE as a positive integer and fail fast on invalid values. This prevents silently disabling the ClickHouse memory limit.

• Bug Fixes
  • Added toPositiveIntegerOrUndefined and used it for CLICKHOUSE_MAX_MEMORY_USAGE.
  • Unset/empty stays undefined; invalid values now throw "CLICKHOUSE_MAX_MEMORY_USAGE must be a positive integer" at startup.
  • Added tests for default, valid, and invalid values in resolve-config.test.ts.

^{Written for commit 0205042. Summary will update on new commits.}