docs: add NatSpec to Script and Test base contracts

[daixihegu]

Summary

• Add NatSpec to the Script and Test base contracts.
• Document the IS_SCRIPT and IS_TEST marker getters.
• Keep the change documentation-only with no behavior changes.

Motivation

This continues the NatSpec coverage work tracked in #653. Script.sol and Test.sol are still listed as missing public-facing NatSpec coverage, and documenting them helps generated forge-std reference docs include the default base contracts and their marker getters.

Part of #653.

Testing

• forge fmt --check src/Script.sol src/Test.sol
• forge build
• forge test --no-match-contract ConfigTest -vvv
• git diff --check