fix: type-safe config parseEnv return types

[AztecBot]

Summary

Fixes two TypeScript type errors introduced by PR #22445 (refactor: type-safe config).

ConfigMappingsType<T> uses Required<T>[K] to derive the parseEnv return type, so for optional number fields, parseEnv must return number — not number | undefined. The undefined case for missing env vars is already handled by the config framework before parseEnv is called.

• stdlib/src/config/sequencer-config.ts: attestationPropagationTime.parseEnv now returns number instead of number | undefined
• p2p/src/config.ts: l1PublishingTime.parseEnv now returns number instead of number | undefined

ClaudeBox log: https://claudebox.work/s/1b406eba6bceb29c?run=1